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Web Services for the Real World 




O'REILLY 



Leonard Richardson & Sam Ruby 
Foreword by David Heinemeier Hansson 



Web Services/Web Development 



O'REILLY 



RESTful Web Services 

^ "Every developer working with the Web needs to read this book. " 
— David Heinenieier Hansson, creator of the Rails framework 
"RESTful Web Services finally provides a practical road map for constructing 
services that embrace the Web, instead of trying to route around it." 

— Adam Trachtenberg, PHP author and eBay Web Services Evangelist 

You've built web sites that can be used by other people. But can you build web sites that are usable 
by machines? The World Wide Web isn't just a big distributed application — it's a platform for 
distributed computing. Web services, mashups, and Ajax applications all use the Web as a platform. 
But today's web service technologies have lost sight of the simplicity that made the Web successful; 
they don't work like the Web, and they're missing out on its advantages. 

This book puts the "Web" back into web services. It explains how you can connect to the program- 
mable web with the technologies you already use every day. The key is REST, the architectural 
style that drives the Web. RESTful Web Services: 

• Emphasizes the power of basic web technologies — the HTTP application protocol, the URI 
naming standard, and the XML markup language 

• Introduces the Resource-Oriented Architecture (ROA), a common-sense set of rules lor designing 
RESTful web services 

• Reveals how a RESTful design is simpler, more versatile, and more scalable than a design 
based on Remote Procedure Calls (RPCs) 

• Includes real-world examples of RESTful web services, like Amazon's Simple Storage Service 
and the Atom Publishing Protocol 

• Discusses web service clients for popular programming languages 

• Demonstrates how to implement RESTful services in three popular frameworks — Ruby on 
Rails, Restlet (for Java), and Django (for Python) 

• Poetises on practical issues, stich as how to design and implement RESTful web services 
and clients 

RESTful Web Services is the first book that applies the REST design philosophy to real web services. 
It sets down the best practices you need to make your design a success and the techniques you need 
to ttirn your design into working code. You can harness the power of the Web for programmable 
applications — you just have to work with the Web instead of against it. This book explains how. 



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RESTful Web Services 



Leonard Richardson and Sam Ruby 



O'REILLY 

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RESTful Web Services 

by Leonard Richardson and Sam Ruby 

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ISBN: 978-0-596-52926-0 
[LSI] 

1302884426 



[2011-04-15] 



For Woot, Moby, and Beet. 
— Leonard 

For Christopher, Catherine, and Carolyn. 
— Sam 



Table of Contents 



Foreword xi 

Preface xiii 

1. The Programmable Web and Its Inhabitants 1 

Kinds of Things on the Programmable Web 4 

HTTP: Documents in Envelopes 5 

Method Information 8 

Scoping Information 1 1 

The Competing Architectures 13 

Technologies on the Programmable Web 18 

Leftover Terminology 20 

2. Writing Web Service Clients 23 

Web Services Are Web Sites 23 

del.icio.us: The Sample Application 26 

Making the Request: HTTP Libraries 29 

Processing the Response: XML Parsers 38 

JSON Parsers: Handling Serialized Data 44 

Clients Made Easy with WADL 47 

3. What Makes RESTful Services Different? 49 

Introducing the Simple Storage Service 49 

Object-Oriented Design of S3 50 

Resources 52 

HTTP Response Codes 54 

An S3 Client 55 

Request Signing and Access Control 64 

Using the S3 Client Library 70 

Clients Made Transparent with ActiveResource 71 

Parting Words 77 



vii 



4. The Resource-Oriented Architecture 79 

Resource-Oriented What Now? 79 

What's a Resource? 81 

URIs 81 

Addressability 84 

Statelessness 86 

Representations 91 

Links and Connectedness 94 

The Uniform Interface 96 

That's It! 105 

5. Designing Read-Only Resource-Oriented Services 107 

Resource Design 108 

Turning Requirements Into Read-Only Resources 109 

Figure Out the Data Set 110 

Split the Data Set into Resources 112 

Name the Resources 117 

Design Your Representations 123 

Link the Resources to Each Other 135 

The HTTP Response 137 

Conclusion 140 

6. Designing Read/Write Resource-Oriented Services 143 

User Accounts as Resources 144 

Custom Places 157 

A Look Back at the Map Service 165 

7. A Service Implementation 167 

A Social Bookmarking Web Service 167 

Figuring Out the Data Set 168 

Resource Design 171 

Design the Representation(s) Accepted from the Client 183 

Design the Representation(s) Served to the Client 184 

Connect Resources to Each Other 185 

What's Supposed to Happen? 186 

What Might Go Wrong? 187 

Controller Code 188 

Model Code 205 

What Does the Client Need to Know? 209 

8. REST and R0A Best Practices 215 

Resource-Oriented Basics 215 



viii | Table of Contents 



The Generic ROA Procedure 216 

Addressability 216 

State and Statelessness 217 

Connectedness 218 

The Uniform Interface 218 

This Stuff Matters 221 

Resource Design 227 

URI Design 233 

Outgoing Representations 234 

Incoming Representations 234 

Service Versioning 235 

Permanent URIs Versus Readable URIs 236 

Standard Features of HTTP 237 

Faking PUT and DELETE 25 1 

The Trouble with Cookies 252 

Why Should a User Trust the HTTP Client? 253 

9. The Building Blocks of Services 259 

Representation Formats 259 

Prepackaged Control Flows 272 

Hypermedia Technologies 284 

10. The Resource-Oriented Architecture Versus Big Web Services 299 

What Problems Are Big Web Services Trying to Solve? 300 

SOAP 300 

WSDL 304 

UDDI 309 

Security 310 

Reliable Messaging 311 

Transactions 312 

BPEL, ESB, and SOA 313 

Conclusion 314 

11. Ajax Applications as REST Clients 315 

From AJAX to Ajax 315 

The Ajax Architecture 316 

A del. icio. us Example 317 

The Advantages of Ajax 320 

The Disadvantages of Ajax 321 

REST Goes Better 322 

Making the Request 323 

Handling the Response 324 

JSON ' 325 



Table of Contents | ix 



Don't Bogart the Benefits of REST 326 
Cross-Browser Issues and Ajax Libraries 327 
Subverting the Browser Security Model 33 1 

12. Frameworks for RESTful Services 339 

Ruby on Rails 339 
Restlet 343 
Django 355 

A. Some Resources for REST and Some RESTful Resources 365 

B. The HTTP Response Code Top 42 371 

C. The HTTP Header Top Infinity 389 

Index 409 



x | Table of Contents 



Foreword 



The world of web services has been on a fast track to supernova ever since the architect 
astronauts spotted another meme to rocket out of pragmatism and into the universe of 
enterprises. But, thankfully, all is not lost. A renaissance of HTTP appreciation is 
building and, under the banner of REST, shows a credible alternative to what the mer- 
chants of complexity are trying to ram down everyone's throats; a simple set of prin- 
ciples that every day developers can use to connect applications in a style native to the 
Web. 

RESTful Web Services shows you how to use those principles without the drama, the 
big words, and the miles of indirection that have scared a generation of web developers 
into thinking that web services are so hard that you have to rely on BigCo implemen- 
tations to get anything done. Every developer working with the Web needs to read this 
book. 

— David Heinemeier Hansson 



xi 



Preface 



A complex system that works is invariably found to have 
evolved from a simple system that worked. 

— John Gall 
Systemantics 

We wrote this book to tell you about an amazing new technology. It's here, it's hot, 
and it promises to radically change the way we write distributed systems. We're talking 
about the World Wide Web. 

Okay, it's not a new technology. It's not as hot as it used to be, and from a technical 
standpoint it's not incredibly amazing. But everything else is true. In 10 years the Web 
has changed the way we live, but it's got more change left to give. The Web is a simple, 
ubiquitous, yet overlooked platform for distributed programming. The goal of this 
book is to pull out that change and send it off into the world. 

It may seem strange to claim that the Web's potential for distributed programming has 
been overlooked. After all, this book competes for shelf space with any number of other 
books about web services. The problem is, most of today's "web services" have nothing 
to do with the Web. In opposition to the Web's simplicity, they espouse a heavyweight 
architecture for distributed object access, similar to COM or CORBA. Today's "web 
service" architectures reinvent or ignore every feature that makes the Web successful. 

It doesn't have to be that way. We know the technologies behind the Web can drive 
useful remote services, because those services exist and we use them every day. We 
know such services can scale to enormous size, because they already do. Consider the 
Google search engine. What is it but a remote service for querying a massive database 
and getting back a formatted response? We don't normally think of web sites as "serv- 
ices," because that's programming talk and a web site's ultimate client is a human, but 
services are what they are. 

Every web application — every web site — is a service. You can harness this power for 
programmable applications if you work with the Web instead of against it, if you don't 
bury its unique power under layers of abstraction. It's time to put the "web" back into 
"web services." 



xiii 



The features that make a web site easy for a web surfer to use also make a web service 
API easy for a programmer to use. To find the principles underlying the design of these 
services, we can just translate the principles for human-readable web sites into terms 
that make sense when the surfers are computer programs. 

That's what we do in this book. Our goal throughout is to show the power (and, where 
appropriate, the limitations) of the basic web technologies: the HTTP application pro- 
tocol, the URI naming standard, and the XML markup language. Our topic is the set 
of principles underlying the Web: Representational State Transfer, or REST. For the 
first time, we set down best practices for "RESTful" web services. We cut through the 
confusion and guesswork, replacing folklore and implicit knowledge with concrete 
advice. 

We introduce the Resource-Oriented Architecture (ROA), a commonsense set of rules 
for designing RESTful web services. We also show you the view from the client side: 
how you can write programs to consume RESTful services. Our examples include real- 
world RESTful services like Amazon's Simple Storage Service (S3), the various incar- 
nations of the Atom Publishing Protocol, and Google Maps. We also take popular 
services that fall short of RESTfulness, like the del.icio.us social bookmarking API, and 
rehabilitate them. 



The Web Is Simple 

Why are we so obsessed with the Web that we think it can do everything? Perhaps we 
are delusional, the victims of hype. The web is certainly the most-hyped part of the 
Internet, despite the fact that HTTP is not the most popular Internet protocol. De- 
pending on who's measuring, the bulk of the world's Internet traffic comes from email 
(thanks to spam) or BitTorrent (thanks to copyright infringement) . If the Internet were 
to disappear tomorrow, email is the application people would miss the most. So why 
the Web? What makes HTTP, a protocol designed to schlep project notes around a 
physics lab, also suited for distributed Internet applications? 

Actually, to say that HTTP was designed for anything is to pay it a pretty big compli- 
ment. HTTP and HTML have been called "the Whoopee Cushion and Joy Buzzer of 
Internet protocols, only comprehensible as elaborate practical jokes" — and that's by 
someone who likes them.*The first version of HTTP sure looked like a joke. Here's a 
sample interaction between client and server: 

Client request Server response 

GET /hello. txt Hello, world! 



Clay Shirky, "In Praise of Evolvable Systems" (http://www.shirky.com/ivritings/evolve.html) 



xiv | Preface 



That's it. You connected to the server, gave it the path to a document, and then the 
server sent you the contents of that document. You could do little else with HTTP 0.9. 
It looked like a featureless rip-off of more sophisticated file transfer protocols like FTP. 

This is, surprisingly, a big part of the answer. With tongue only slightly in cheek we 
can say that HTTP is uniquely well suited to distributed Internet applications because 
it has no features to speak of. You tell it what you want, and it gives it to you. In a twist 
straight out of a kung-fu movie,+HTTP's weakness is its strength, its simplicity its 
power. 

In that first version of HTTP, cleverly disguised as a lack of features, we can see ad- 
dressability and statelessness: the two basic design decisions that made HTTP an im- 
provement on its rivals, and that keep it scalable up to today's mega-sites. Many of the 
features lacking in HTTP 0.9 have since turned out to be unnecessary or counterpro- 
ductive. Adding them back actually cripples the Web. Most of the rest were imple- 
mented in the 1 .0 and 1 . 1 revisions of the protocol. The other two technologies essential 
to the success of the Web, URIs and HTML (and, later, XML), are also simple in im- 
portant senses. 

Obviously, these "simple" technologies are powerful enough to give us the Web and 
the applications we use on it. In this book we go further, and claim that the World 
Wide Web is a simple and flexible environment for distributed programming. We also 
claim to know the reason for this: that there is no essential difference between the 
human web designed for our own use, and the "programmable web" designed for con- 
sumption by software programs. We say: if the Web is good enough for humans, it's 
good enough for robots. We just need to make some allowances. Computer programs 
are good at building and parsing complex data structures, but they're not as flexible as 
humans when it comes to interpreting documents. 

Big Web Services Are Not Simple 

There are a number of protocols and standards, mostly built on top of HTTP, designed 
for building Web Services (note the capitalization). These standards are collectively 
called the WS-* stack. They include WS-Notification, WS-Security, WSDL, and SOAP. 
Throughout this book we give the name "Big Web Services" to this collection of tech- 
nologies as a fairly gentle term of disparagement. 

This book does not cover these standards in any great detail. We believe you can im- 
plement web services without implementing Big Web Services: that the Web should 
be all the service you need. We believe the Web's basic technologies are good enough 
to be considered the default platform for distributed services. 

Some of the WS-* standards (such as SOAP) can be used in ways compatible with REST 
and our Resource-Oriented Architecture. In practice, though, they're used to 

■f Legend of The Drunken Protocol (1991) 



Preface | xv 



implement Remote Procedure Call applications over HTTP. Sometimes an RPC style 
is appropriate, and sometimes other needs take precedence over the virtues of the Web . 
This is fine. 

What we don't like is needless complexity. Too often a programmer or a company 
brings in Big Web Services for a job that plain old HTTP could handle just fine. The 
effect is that HTTP is reduced to a transport protocol for an enormous XML payload 
that explains what's "really" going on. The resulting service is far too complex, im- 
possible to debug, and won't work unless your clients have the exact same setup as you 
do. 

Big Web Services do have one advantage: modern tools can create a web service from 
your code with a single click, especially if you're developing in Java or C#. If you're 
using these tools to generate RPC-style web services with the WS-* stack, it probably 
doesn't matter to you that a RESTful web service would be much simpler. The tools 
hide all the complexity, so who cares? Bandwidth and CPU are cheap. 

This attitude works when you're working in a homogeneous group, providing services 
behind a firewall for other groups like yours. If your group has enough political clout, 
you may be able to get people to play your way outside the firewall. But if you want 
your service to grow to Internet scale, you'll have to handle clients you never planned 
for, using custom-built software stacks to do things to your service you never imagined 
were possible. Your users will want to integrate your service with other services you've 
never heard of. Sound difficult? This already happens on the Web every day. 

Abstractions are never perfect. Every new layer creates failure points, interoperability 
hassles, and scalability problems. New tools can hide complexity, but they can't justify 
it — and they always add it. Getting a service to work with the Web as a whole means 
paying attention to adaptability, scalability, and maintainability. Simplicity — that de- 
spised virtue of HTTP 0.9 — is a prerequisite for all three. The more complex the system, 
the more difficult it is to fix when something goes wrong. 

If you provide RESTful web services, you can spend your complexity on additional 
features, or on making multiple services interact. Success in providing services also 
means being part of the Web instead of just "on" the Web: making your information 
available under the same rules that govern well-designed web sites. The closer you are 
to the basic web protocols, the easier this is. 

The Story of the REST 

REST is simple, but it's well defined and not an excuse for implementing web services 
as half-assed web sites because "they're the same." Unfortunately, until now the main 
REST reference was chapter five of Roy Fielding's 2000 Ph.D. dissertation, which is a 
good read for a Ph.D. dissertation, but leaves most of the real-world questions unan- 
swered. iThat's because it presents REST not as an architecture but as a way of judging 
architectures. The term "RESTful" is like the term "object-oriented." A language, a 



xvi | Preface 



framework, or an application may be designed in an object-oriented way, but that 
doesn't make its architecture the object-oriented architecture. 

Even in object-oriented languages like C++ and Ruby, it's possible to write programs 
that are not truly object-oriented. HTTP in the abstract does very well on the criteria 
of REST. (It ought to, since Fielding co-wrote the HTTP standard and wrote his dis- 
sertation to describe the architecture of the Web.) But real web sites, web applications, 
and web services often betray the principles of REST. How can you be sure you're 
correctly applying the principles to the problem of designing a specific web service? 

Most other sources of information on REST are informal: mailing lists, wikis, and 
weblogs (I list some of the best in Appendix A) . Up to now, REST's best practices have 
been a matter of folklore. What's needed is a concrete architecture based on the REST 
meta-architecture: a set of simple guidelines for implementing typical services that ful- 
fill the potential of the Web. We present one such architecture in this book as the 
Resource-Oriented Architecture (see Chapter 4). It's certainly not the only possible 
high-level RESTful architecture, but we think it's a good one for designing web services 
that are easy for clients to use. 

We wrote the ROA to bring the best practices of web service design out of the realm 
of folklore. What we've written is a suggested baseline. If you've tried to figure out 
REST in the past, we hope our architecture gives you confidence that what you're doing 
is "really" REST. We also hope the ROA will help the community as a whole make 
faster progress in coming up with and codifying best practices. We want to make it easy 
for programmers to create distributed web applications that are elegant, that do the job 
they're designed for, and that participate in the Web instead of merely living on top of 
it. 

We know, however, that it's not enough to have all these technical facts at your dis- 
posal. We've both worked in organizations where major architectural decisions didn't 
go our way. You can't succeed with a RESTful architecture if you never get a chance 
to use it. In addition to the technical know-how, we must give you the vocabulary to 
argue for RESTful solutions. We've positioned the ROA as a simple alternative to the 
RPC-style architecture used by today's SOAP+WSDL services. The RPC architecture 
exposes internal algorithms through a complex programming-language-like interface 
that's different for every service. The ROA exposes internal data through a simple 
document-processing interface that's always the same. In Chapter 10, we compare the 
two architectures and show how to argue for the ROA. 



t Fielding, Roy Thomas. Architectural Styles and the Design of Network-Based Software Architectures, Doctoral 
dissertation, University of California, Irvine, 2000 (http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm) 



Preface | xvii 



Reuniting the Webs 



Programmers have been using web sites as web services for years — unofficially, of 
course. §It's difficult for a computer to understand web pages designed for human con- 
sumption, but that's never stopped hackers from fetching pages with automated clients 
and screen-scraping the interesting bits. Over time, this drive was sublimated into pro- 
grammer-friendly technologies for exposing a web site's functionality in officially sanc- 
tioned ways — RSS, XML-RPC, and SOAP. These technologies formed a programmable 
web, one that extended the human web for the convenience of software programs. 

Our ultimate goal in this book is to reunite the programmable web with the human 
web. We envision a single interconnected network: a World Wide Web that runs on 
one set of servers, uses one set of protocols, and obeys one set of design principles. A 
network that you can use whether you're serving data to human beings or computer 
programs. 

The Internet and the Web did not have to exist. They come to us courtesy of misallo- 
cated defense money, skunkworks engineering projects, worse-is-better engineering 
practices, big science, naive liberal idealism, cranky libertarian politics, techno- 
fetishism, and the sweat and capital of programmers and investors who thought they'd 
found an easy way to strike it rich. 

The result is, amazingly, a simple, open (for now), almost universal platform for net- 
worked applications. This platform contains much of human knowledge and supports 
most fields of human endeavor. We think it's time to seriously start applying its rules 
to distributed programming, to open up that information and those processes to au- 
tomatic clients. If you agree, this book will show you how to do it. 

What's in This Book? 

In this book we focus on practical issues: how to design and implement RESTful web 
services, and clients for those services. Our secondary focus is on theory: what it means 
to be RESTful, and why web services should be more RESTful instead of less. We don't 
cover everything, but we try to hit today's big topics, and because this is the first book 
of its kind, we return to the core issue — how to design a RESTful service — over and 
over again. 

The first three chapters introduce web services from the client's perspective and show 
what's special about RESTful services. 



§ For an early example, see Jon Udell's 1996 Byte article "On-Line Componentware" (http://www.byte.com/ 
art/9611/sec9/artl.htm) . Note: "A powerful capability for ad hoc distributed computing arises naturally from 
the architecture of the Web." That's from 1996, folks. 



xviii | Preface 



Chapter 1 , The Programmable Web and Its Inhabitants 

In this chapter we introduce web services in general: programs that go over the 
Web and ask a foreign server to provide data or run an algorithm. We demonstrate 
the three common web service architectures: RESTful, RPC-style, and REST-RPC 
hybrid. We show sample HTTP requests and responses for each architecture, along 
with typical client code. 

Chapter 2, Writing Web Service Clients 

In this chapter we show you how to write clients for existing web services, using 
an HTTP library and an XML parser. We introduce a popular REST-RPC service 
(the web service for the social bookmarking site del.icio.us) and demonstrate cli- 
ents written in Ruby, Python, Java, C#, and PHP. We also give technology rec- 
ommendations for several other languages, without actually showing code. Java- 
Script and Ajax are covered separately in Chapter 11. 

Chapter 3, What Makes RESTful Services Different? 

We take the lessons of Chapter 2 and apply them to a purely RESTful service: 
Amazon's Simple Storage Service (S3). While building an S3 client we illustrate 
some important principles of REST: resources, representations, and the uniform 
interface. 

The next six chapters form the core of the book. They focus on designing and imple- 
menting your own RESTful services. 

Chapter 4, The Resource-Oriented Architecture 

A formal introduction to REST, not in its abstract form but in the context of a 
specific architecture for web services. Our architecture is based on four important 
REST concepts: resources, their names, their representations, and the links be- 
tween them. Its services should be judged by four RESTful properties: addressa- 
bility, statelessness, connectedness, and the uniform interface. 

Chapter 5, Designing Read-Only Resource-Oriented Services 

We present a procedure for turning an idea or a set of requirements into a set of 
RESTful resources. These resources are read-only: clients can get data from your 
service but they can't send any data of their own. We illustrate the procedure by 
designing a web service for serving navigable maps, inspired by the Google Maps 
web application. 

Chapter 6, Designing Read/Write Resource-Oriented Services 

We extend the procedure from the previous chapter so that clients can create, 
modify, and delete resources. We demonstrate by adding two new kinds of re- 
source to the map service: user accounts and user-defined places. 

Chapter 7, A Service Implementation 

We remodel an RPC-style service (the del.icio.us REST-RPC hybrid we wrote cli- 
ents for back in Chapter 2) as a purely RESTful service. Then we implement that 
service as a Ruby on Rails application. Fun for the whole family! 



Preface | xix 



Chapter 8, REST and ROA Best Practices 

In this chapter we collect our earlier suggestions for service design into one place, 
and add new suggestions. We show how standard features of HTTP can help you 
with common problems and optimizations. We also give resource-oriented designs 
for tough features like transactions, which you may have thought were impossible 
to do in RESTful web services. 

Chapter 9, The Building Blocks of Services 

Here we describe extra technologies that work on top of REST's big three of HTTP, 
URI, and XML. Some of these technologies are file formats for conveying state, like 
XHTML and its microformats. Some are hypermedia formats for showing clients 
the levers of state, like WADL. Some are sets of rules for building RESTful web 
services, like the Atom Publishing Protocol. 

The last three chapters cover specialized topics, each of which could make for a book 
in its own right: 

Chapter 10, The Resource-Oriented Architecture Versus Big Web Services 

We compare our architecture, and REST in general, to another leading brand. We 
think that RESTful web services are simpler, more scalable, easier to use, better 
attuned to the philosophy of the Web, and better able to handle a wide variety of 
clients than are services based on SOAP, WSDL, and the WS-* stack. 

Chapter 11, Ajax Applications as REST Clients 

Here we explain the Ajax architecture for web applications in terms of web services: 
an Ajax application is just a web service client that runs inside your web browser. 
That makes this chapter an extension of Chapter 2. We show how to write clients 
for RESTful web services using XHLHttpRequest and the standard JavaScript library. 

Chapter 12, Frameworks for RESTful Services 

In the final chapter we cover three popular frameworks that make it easy to im- 
plement RESTful web services: Ruby on Rails, Restlet (for Java), and Django (for 
Python). 

We also have three appendixes we hope you find useful: 

Appendix A, Some Resources for REST and Some RESTful Resources 

The first part lists interesting standards, tutorials, and communities related to 
RESTful web services. The second part lists some existing, public RESTful web 
services that you can use and learn from. 

Appendix B, The HTTP Response Code Top 42 

Describes every standard HTTP response code (plus one extension), and explains 
when you'd use each one in a RESTful web service. 

Appendix C, The HTTP Header Top Infinity 

Does the same thing for HTTP headers. It covers every standard HTTP header, 
and a few extension headers that are useful for web services. 



xx | Preface 



Which Parts Should You Read? 

We organized this book for the reader who's interested in web services in general: 
someone who learns by doing, but who doesn't have much experience with web serv- 
ices. If that describes you, the simplest path through this book is the best. You can start 
at the beginning, read through Chapter 9, and then read onward as you're interested. 

If you have more experience, you might take a different path through the book. If you're 
only concerned with writing clients for existing services, you'll probably focus on 
Chapters 1, 2, 3, and 11 — the sections on service design won't do you much good. If 
you want to create your own web service, or you're trying to figure out what REST 
really means, you might start reading from Chapter 3. If you want to compare REST 
to the WS-* technologies, you might start by reading Chapters 1, 3, 4, and 10. 



Administrative Notes 

This book has two authors (Leonard and Sam), but for the rest of the book we'll be 
merging our identities into a single authorial "I." In the final chapter (Chapter 12), the 
authorial "I" gets a little bit more crowded, as Django and Restlet developers join in to 
show how their frameworks let you build RESTful services. 

We assume that you're a competent programmer, but not that you have any experience 
with web programming in particular. What we say in this book is not tied to any pro- 
gramming language, and we include sample code for RESTful clients and services in a 
variety of languages. But whenever we're not demonstrating a specific framework or 
language, we use Ruby (http://www.ruby-lang.org/) as our implementation language. 

We chose Ruby because it's concise and easy to read, even for programmers who don't 
know the language. (And because it's nice and confusing in conjunction with Sam's 
last name.) Ruby's standard web framework, Ruby on Rails, is also one of the leading 
implementation platforms for RESTful web services. If you don't know Ruby, don't 
worry: we include lots of comments explaining Ruby-specific idioms. 

The sample programs in this book are available for download from this book's official 
web site (http://www.oreilly.com/catalog/9780596529260). This includes the entire 
Rails application from Chapter 7, and the corresponding Restlet and Django applica- 
tions from Chapter 12. It also includes Java implementations of many of the clients 
that only show up in the book as Ruby implementations. These client programs use 
the Restlet library, and were written by Restlet developers Jerome Louvel and Dave 
Pawson. If you're more familiar with Java than with Ruby, these implementations may 
help you grasp the concepts behind the code. Most notably, there's a full Java imple- 
mentation of the Amazon S3 client from Chapter 3 in there. 



Preface | xxi 



Conventions Used in This Book 



The following typographical conventions are used in this book: 
Italic 

Indicates new terms, URLs, email addresses, filenames, and file extensions. 
Constant width 

Used for program listings, as well as within paragraphs to refer to program elements 
such as variable or function names, databases, data types, environment variables, 
statements, and keywords. 

Constant width bold 

Shows commands or other text that should be typed literally by the user. 

Constant width italic 

Shows text that should be replaced with user-supplied values or by values deter- 
mined by context. 



This book is here to help you get your job done. In general, you may use the code in 
this book in your programs and documentation. You do not need to contact us for 
permission unless you're reproducing a significant portion of the code. For example, 
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We appreciate, but do not require, attribution. An attribution usually includes the title, 
author, publisher, and ISBN. For example: "RESTful Web Services by Leonard Ri- 
chardson and Sam Ruby. Copyright 2007 O'Reilly Media, Inc., 978-0-596-52926-0." 

If you feel your use of code examples falls outside fair use or the permission given above, 
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xxii | Preface 



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Acknowledgments 

We're ultimately indebted to the people whose work made us see that we could pro- 
gram directly with HTTP. For Sam, it was Rael Dornfest with his Blosxom blogging 
application. Leonard's experience stems from building screen-scraping applications in 
the mid-90s. His thanks go to those whose web design made their sites usable as web 
services: notably, the pseudonymous author of the online comic "Pokey the Penguin." 

Once we had this insight, Roy Fielding was there to flesh it out. His thesis named and 
defined something that was for us only a feeling. Roy's theoretical foundation is what 
we've tried to build on. 



Preface | xxiii 



In writing this book we had an enormous amount of help from the REST community. 
We're grateful for the feedback we got from Benjamin Carlyle, David Gourley, Joe 
Gregorio, Marc Hadley, Chuck Hinson, Pete Lacey, Larry Liberto, Benjamin Pollack, 
Aron Roberts, Richard Walker, and Yohei Yamamoto. Others helped us unknowingly, 
through their writings: Mark Baker, Tim Berners-Lee, Alex Bunardzic, Duncan Cragg, 
David Heinemeier Hansson, Ian Hickson, Mark Nottingham, Koranteng Ofosu- 
Amaah, Uche Ogbuji, Mark Pilgrim, Paul Prescod, Clay Shirky, Brian Totty, and Jon 
Udell. Of course, all opinions in this book, and any errors and omissions, are our own. 

Our editor Michael Loukides was helpful and knowledgeable throughout the process 
of developing this book. We'd also like to thank Laurel Ruma and everyone else at 
O'Reilly for their production work. 

Finally, Jerome Louvel, Dave Pawson, and Jacob Kaplan-Moss deserve special thanks. 
Their knowledge of Restlet and Django made Chapter 12 possible. 



xxiv | Preface 



CHAPTER 1 

The Programmable Web and Its 

Inhabitants 



When you write a computer program, you're not limited to the algorithms you can 
think up. Your language's standard library gives you some algorithms. You can get 
more from books, or in third -party libraries you find online. Only if you're on the very 
cutting edge should you have to come up with your own algorithms. 

If you're lucky, the same is true for data. Some applications are driven entirely by the 
data the users type in. Sometimes data just comes to you naturally: if you're analyzing 
spam, you should have no problem getting all you need. You can download a few public 
data sets — word lists, geographical data, lists of prime numbers, public domain texts 
— as though they were third-party libraries. But if you need some other kind of data, it 
doesn't look good. Where's the data going to come from? More and more often, it's 
coming from the programmable web. 

When you — a human being — want to find a book on a certain topic, you probably 
point your web browser to the URI of an online library or bookstore: say, http:// 
www.amazon.com/. 

The common term for the address of something on the Web is "URL." 
I say "URI" throughout this book because that's what the HTTP stand- 
ard says. Every URI on the Web is also a URL, so you can substitute 
"URL" wherever I say "URI" with no loss of meaning. 

You're served a web page, a document in HTML format that your browser renders 
graphically. You visually scan the page for a search form, type your topic (say, "web 
services") into a text box, and submit the form. At this point your web browser makes 
a second HTTP request, to a URI that incorporates your topic. To continue the Amazon 
example, the second URI your browser requests would be something like http:lla.ma 
zon.com/s?url=search-alias%3Dstripbooks&field-keywords=web+services. 




1 



The web server at amazon . com responds by serving a second document in HTML format. 
This document contains a description of your search results, links to additional search 
options, and miscellaneous commercial enticements (see Example 1-1). Again, your 
browser renders the document in graphical form, and you look at it and decide what 
to do from there. 

Example 1-1. Part of the HTML response from amazon.com 

<a href =" http://www.amazon.com/Restful-Web- Services -Leonard- Richardson/dp/. . .> 
<span class="srTitle">RESTful Web Services</span> 
</a> 

by Leonard Richardson and Sam Ruby 

<span class="bindingBlock"> 

(<span class="binding">Paperback</span> - May 1, 2007) 
</span> 

The Web you use is full of data: book information, opinions, prices, arrival times, 
messages, photographs, and miscellaneous junk. It's full of services: search engines, 
online stores, weblogs, wikis, calculators, and games. Rather than installing all this data 
and all these programs on your own computer, you install one program — a web browser 
— and access the data and services through it. 

The programmable web is just the same. The main difference is that instead of arranging 
its data in attractive HTML pages with banner ads and cute pastel logos, the program- 
mable web usually serves stark, brutal XML documents. The programmable web is not 
necessarily for human consumption. Its data is intended as input to a software program 
that does something amazing. 

Example 1-2 shows a Ruby script that uses the programmable web to do a traditional 
human web task: find the titles of books matching a keyword. It hides the web access 
under a programming language interface, using the Ruby/Amazon library (http:// 
www.caliban.org/ruby/ruby-amazon.shtml). 

Example 1-2. Searching for books with a Ruby script 

#!/usr/bin/ruby -w 

# amazon-book-search.rb 
require 'amazon/search' 

if ARCV.size != 2 

puts "Usage: #{$0} [Amazon Web Services AccessKey ID] [text to search for]" 
exit 
end 

access_key, search_request = ARCV 

req = Amazon: :Search: :Request.new(access_key) 

# For every book in the search results... 

req.keyword_search(search_request, 'books', Amazon: : Search :: LIGHT) do | book | 
# Print the book's name and the list of authors. 



2 | Chapter 1: The Programmable Web and Its Inhabitants 



puts %{"#{book.product_name}" by #{book. authors. join( ' , ')}} 
end 

To run this program, you'll need to sign up for an Amazon Web Services account 
(http://aws.amazon.com/) and customize the Ruby code with your Access Key ID. 
Here's a sample run of the program: 

$ ruby amazon-search.rb C1D4NQS41IMK2 "restful web services" 

"RESTful Web Services" by Leonard Richardson, Sam Ruby 

"Hacking with Ruby: Ruby and Rails for the Real World" by Mark Watson 

At its best, the programmable web works the same way as the human web. When 
amazon-book-search.rb calls the method Amazon: : Search : :Request#keyword_search, 
the Ruby program starts acting like a web browser. It makes an HTTP request to a URI: 
in this case, something like http://xml.amazon.com/onca/xml3?KeywordSearch=restful 
+web+services&mode=books&f=xml&type=lite&page=l. The web server at xml.ama 
zon.com responds with an XML document. This document, shown in Example 1-3, 
describes the search results, just like the HTML document you see in your web browser, 
but in a more structured form. 

Example 1-3. Part of the XML response from xml.amazon.com 

<ProductName>RESTful Web Services</ProductName> 

<Catalog>Book</Catalog> 

<Authors> 

<Author> Leonard Richardson</Author> 

<Author>Sam Ruby</Author> 
</Authors> 

<ReleaseDate>01 May, 2007</ReleaseDate> 

Once a web browser has submitted its HTTP request, it has a fairly easy task. It needs 
to render the response in a way a human being can understand. It doesn't need to figure 
out what the HTTP response means: that's the human's job. A web service client 
doesn't have this luxury. It's programmed in advance, so it has to be both the web 
browser that fetches the data, and the "human" who decides what the data means. Web 
service clients must automatically extract meaning from HTTP responses and make 
decisions based on that meaning. 

In Example 1-2, the web service client parses the XML document, extracts some inter- 
esting information (book titles and authors), and prints that information to standard 
output. The program amazon-book-search.rb is effectively a small, special-purpose web 
browser, relaying data to a human reader. It could easily do something else with the 
Amazon book data, something that didn't rely on human intervention at all: stick the 
book titles into a database, maybe, or use the author information to drive a recom- 
mendation engine. 

And the data doesn't have to always flow toward the client. Just as you can bend parts 
of the human web to your will (by posting on your weblog or buying a book), you can 



The Programmable Web and Its Inhabitants | 3 



write clients that modify the programmable web. You can use it as a storage space or 
as another source of algorithms you don't have to write yourself. It depends on what 
service you need, and whether you can find someone else to provide it. 

Example 1 -4 is an example of a web service client that modifies the programmable web : 
the s3sh command shell for Ruby (http://amazon.rubyforge.org/) . It's one of many cli- 
ents written against another of Amazon's web services: S3, or the Simple Storage Serv- 
ice (http://aws.amazon.com/s3). In Chapter 3 I cover S3's workings in detail, so if you're 
interested in using s3sh for yourself, you can read up on S3 there. 

To understand this s3sh transcript, all you need to know is that Amazon S3 lets its 
clients store labelled pieces of data ("objects") in labelled containers ("buckets"). The 
s3sh program builds an interactive programming interface on top of S3. Other clients 
use S3 as a backup tool or a web host. It's a very flexible service. 

Example 1-4. Manipulating the programmable web with s3sh and S3 
$ s3sh 

>> Service. buckets. collect { |b| b.name } 

=> ["example.com"] 

>> my_bucket = Bucket. find ("example. com") 

>> contents = open("disk_file.txt") .read 

=> "This text is the contents of the file disk_file.txt" 

>> S30bject.store("mydir/mydocument.txt", contents, my_bucket.name) 

>> my_bucket[ 'directory/document. txt' ] .value 

=> "This text is the contents of the file disk_file.txt" 

In this chapter I survey the current state of the programmable web. What technologies 
are being used, what architectures are they used to implement, and what design styles 
are the most popular? I show some real code and some real HTTP conversations, but 
my main goal in this chapter is to get you thinking about the World Wide Web as a 
way of connecting computer programs to each other, on the same terms as it connects 
human beings to each other. 

Kinds of Things on the Programmable Web 

The programmable web is based on HTTP and XML. Some parts of it serve HTML, 
JavaScript Object Notation (JSON), plain text, or binary documents, but most parts 
use XML. And it's all based on HTTP: if you don't use HTTP, you're not on the 
web. 'Beyond that small island of agreement there is little but controversy. The 
terminology isn't set, and different people use common terms (like "REST," the topic 
of this book) in ways that combine into a vague and confusing mess. What's missing 
is a coherent way of classifying the programmable web . With that in place, the meanings 
of individual terms will become clear. 



4 | Chapter 1: The Programmable Web and Its Inhabitants 



Imagine the programmable web as an ecosystem, like the ocean, containing many kinds 
of strange creatures. Ancient scientists and sailors classified sea creatures by their su- 
perficial appearance: whales were lumped in with the fish. Modern scientists classify 
animals according to their position in the evolutionary tree of all life: whales are now 
grouped with the other mammals. There are two analogous ways of classifying the 
services that inhabit the programmable web: by the technologies they use (URIs, SOAP, 
XML-RPC, and so on), or by the underlying architectures and design philosophies. 

Usually the two systems for classifying sea creatures get along. You don't need to do 
DNA tests to know that a tuna is more like a grouper than a sea anenome. But if you 
really want to understand why whales can't breathe underwater, you need to stop clas- 
sifying them as fish (by superficial appearance) and start classifying them as mammals 
(by underlying architecture). + 

When it comes to classifying the programmable web, most of today's terminology sorts 
services by their superficial appearances: the technologies they use. These classifica- 
tions work in most cases, but they're conceptually lacking and they lead to whale-fish 
mistakes. I'm going to present a taxonomy based on architecture, which shows how 
technology choices follow from underlying design principles. I'm exposing divisions 
I'll come back to throughout the book, but my main purpose is to zoom in on the parts 
of the programmable web that can reasonably be associated with the term "REST." 

HTTP: Documents in Envelopes 

If I was classifying marine animals I'd start by talking about the things they have in 
common: DNA, cellular structure, the laws of embryonic development. Then I'd show 
how animals distinguish themselves from each other by specializing away from the 
common ground. To classify the programmable web, I'd like to start off with an over- 
view of HTTP, the protocol that all web services have in common. 

HTTP is a document-based protocol, in which the client puts a document in an enve- 
lope and sends it to the server. The server returns the favor by putting a response docu- 
ment in an envelope and sending it to the client. HTTP has strict standards for what 
the envelopes should look like, but it doesn't much care what goes inside. Exam- 
ple 1-5 shows a sample envelope: the HTTP request my web browser sends when I visit 



Thanks to Big Web Services' WS-Addressing standard, it's now possible to create a web service that's not on 
the Web: one that uses email or TCP as its transport protocol instead of HTTP. I don't think absolutely 
everything has to be on the Web, but it does seem like you should have to call this bizarre spectacle something 
other than a web service. This point isn't really important, since in practice nearly everyone uses HTTP. Thus 
the footnote. The only exceptions I know of are eBay's web services, which can send you SOAP documents 
over email as well as HTTP. 

"f Melville, in Moby-Dick, spends much of Chapter 22 ("Cetology") arguing that the whale is a fish. This sounds 
silly but he's not denying that whales have lungs and give milk; he's arguing for a definition of "fish" based 
on appearance, as opposed to Linnaeus's definition "from the law of nature" (ex lege naturae). 



HTTP: Documents in Envelopes | 5 



the homepage of oreilly . com. I've truncated two lines to make the text fit on the printed 
page. 

Example 1-5. An HTTP GET request for http://www.oreilly.com/index.html 

GET /index. html HTTP/l.l 
Host: www.oreilly.com 

User-Agent: Mozilla/5.0 (Xll; U; Linux i686; en-US; rv:1.7.l2) . . . 

Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9, . . . 

Accept-Language: us,en;q=0.5 

Accept-Encoding: gzip, deflate 

Accept-Charset: ISO-8859-15,utf-8;q=0.7,*;q=0.7 

Keep- Alive: 300 

Connection: keep-alive 

In case you're not familiar with HTTP, now is a good time to point out the major parts 
of the HTTP request. I use these terms throughout the book. 

The HTTP method 

In this request, the method is "GET." In other discussions of REST you may see 
this called the "HTTP verb" or "HTTP action." 

The name of the HTTP method is like a method name in a programming language: 
it indicates how the client expects the server to process this envelope. In this case, 
the client (my web browser) is trying to GET some information from the server 
(www.oreilly.com). 

The path 

This is the portion of the URI to the right of the hostname: here, http://www.oreU 
ly.com/index.html becomes '7index.html. " In terms of the envelope metaphor, the 
path is the address on the envelope. In this book I sometimes refer to the "URI" as 
shorthand for just the path. 

The request headers 

These are bits of metadata: key-value pairs that act like informational stickers 
slapped onto the envelope. This request has eight headers: Host, User-Agent, 
Accept, and so on. There's a standard list of HTTP headers (see Appendix C), and 
applications can define their own. 

The entity-body, also called the document or representation 

This is the document that inside the envelope. This particular request has no entity- 
body, which means the envelope is empty! This is typical for a GET request, where 
all the information needed to complete the request is in the path and the headers. 

The HTTP response is also a document in a envelope. It's almost identical in form to 
the HTTP request. Example 1-6 shows a trimmed version of what the server at 
oreilly.com sends my web browser when I make the request in Example 1-5. 

Example 1-6. The response to an HTTP GET request for http://www.oreilly.com/index.html 
HTTP/l.l 200 OK 

Date: Fri, 17 Nov 2006 15:36:32 GMT 



6 I Chapter 1: The Programmable Web and Its Inhabitants 



Server: Apache 

Last-Modified: Fri, 17 Nov 2006 09:05:32 GMT 
Etag: "7359b7-a7fa-455d8264 
Accept-Ranges: bytes 
Content- Length: 43302 
Content-Type: text/html 
X-Cache: MISS from www.oreilly.com 
Keep-Alive: timeout=15, max=1000 
Connection: Keep-Alive 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 

"http://www.w3.org/TR/xhtmll/DTD/xhtmll-transitional.dtd"> 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en" lang="en"> 
<head> 

<title>oreilly.com -- Welcome to O'Reilly Media, Inc.</title> 



The response can be divided inLo Lhree parts: 

The HTTP response code 

This numeric code tells the client whether its request went well or poorly, and how 
the client should regard this envelope and its contents. In this case the GET oper- 
ation must have succeeded, since the response code is 200 ("OK"). I describe the 
HTTP response codes in Appendix B. 

The response headers 

Just as with the request headers, these are informational stickers slapped onto the 
envelope. This response has 11 headers: Date, Server, and so on. 

The entity-body or representation 

Again, this is the document inside the envelope, and this time there actually is one! 
The entity-body is the fulfillment of my GET request. The rest of the response is 
just an envelope with stickers on it, telling the web browser how to deal with the 
document. 

The most important of these stickers is worth mentioning separately. The response 
header Content-Type gives the media type of the entity-body. In this case, the media 
type is text/html. This lets my web browser know it can render the entity-body as 
an HTML document: a web page. 

There's a standard list of media types (http://www.iana.org/assignments/media- 
typesl). The most common media types designate textual documents (text/html), 
structured data documents (application/xml), and images (image/jpeg). In other 
discussions of REST or HTTP, you may see the media type called the "MIME type," 
"content type," or "data type." 



HTTP: Documents in Envelopes | 7 



Method Information 



HTTP is the one thing that all "animals" on the programmable web have in common. 
Now I'll show you how web services distinguish themselves from each other. There are 
two big questions that today's web services answer differently. If you know how a web 
service answers these questions, you'll have a good idea of how well it works with the 
Web. 

The first question is how the client can convey its intentions to the server. How does 
the server know a certain request is a request to retrieve some data, instead of a request 
to delete that same data or to overwrite it with different data? Why should the server 
do this instead of doing that? 

I call the information about what to do with the data the method information. One way 
to convey method information in a web service is to put it in the HTTP method. Since 
this is how RESTful web services do it, I'll have a lot more to say about this later. For 
now, note that the five most common HTTP methods are GET, HEAD, PUT, DELETE, 
and POST. This is enough to distinguish between "retrieve some data" (GET), "delete 
that same data" (DELETE), and "overwrite it with different data" (PUT). 

The great advantage of HTTP method names is that they're standardized. Of course, 
the space of HTTP method names is much more limited than the space of method 
names in a programming language. Some web services prefer to look for application- 
specific method names elsewhere in the HTTP request: usually in the URI path or the 
request document. 

Example 1-7 is a client for a web service that keeps its method information in the path: 
the web service for Flickr, Yahoo! 's online photo-sharing application. This sample ap- 
plication searches Flickr for photos. To run this program, you'll need to create a Flickr 
account and apply for an API key (http://www.flickr.com/services/api/keys/apply/). 

Example 1-7. Searching Flickr for pictures 

#!/usr/bin/ruby -w 

# flickr-photo-search.rb 
require 'open-uri' 
require ' rexml/document ' 

# Returns the URI to a small version of a Flickr photo, 
def small_photo_uri (photo) 

server = photo. attribute( ' server' ) 

id = photo. attribute('id') 

secret = photo. attribute( ' secret ' ) 

return "http : //static. flickr.com/#{server}/#{id}_#{secret}_m. jpg" 
end 

# Searches Flickr for photos matching a certain tag, and prints a URI 

# for each search result. 

def print_each_photo(api_key, tag) 
# Build the URI 

uri = "http://www. flickr. com/services/rest?method=flickr. photos. search" + 



8 | Chapter 1: The Programmable Web and Its Inhabitants 



"&api_key=#{api_key}&tags=#{tag}" 

# Make the HTTP request and get the entity-body, 
response = open(uri) .read 

# Parse the entity-body as an XML document, 
doc = REXML: : Document. new(response) 

# For each photo found... 

REXML: : XPath. each(doc, '//photo') do | photo | 
# ...generate and print its URI 
puts small_photo_uri(photo) if photo 
end 
end 

# Main program 
# 

if ARGV.size < 2 

puts "Usage: #{$0} [Flickr API key] [search term]" 

exit 
end 

api_key, tag = ARCV 
print_each_photo(api_key, tag) 



XPath: The Bluffer's Guide 

XPath is a domain-specific language for slicing up XML documents without writing a 
lot of code. It has many intimidating features, but you can get by with just a little bit 
of knowledge. The key is to think of an XPath expression as a rule for extracting tags 
or other elements from an XML document. There aren't many XPath expressions in 
this book, but I'll explain every one I use. 

To turn an XPath expression into English, read it from right to left. The expres- 
sion //photo means: 

Find every photo tag photo 
no matter where it is in the document. // 

The Ruby code REXML : : XPath . each (doc, ' // photo ' ) is a cheap way to iterate over every 
photo tag without having to traverse the XML tree. 



This program makes HTTP requests to URIs like http://www.flickr.com/services/rest? 
method=flickr. photos. search&api_key=xxx&tag=penguins. How does the server know 
what the client is trying to do? Well, the method name is pretty clearly 
flickr . photos . search. Except: the HTTP method is GET, and I am getting information, 
so it might be that the method thing is a red herring. Maybe the method information 
really goes in the HTTP action. 



Method Information | 9 



This hypothesis doesn't last for very long, because the Flickr API supports many meth- 
ods, not just "get"-type methods such as flickr. photos, search and 
flickr. people. findBy Email, but also methods like flickr. photos. addTags, 
flickr. photos. comments. deleteComment, and so on. All of them are invoked with an 
HTTP GET request, regardless of whether or not they "get" any data. It's pretty clear 
that Flickr is sticking the method information in the method query variable, and ex- 
pecting the client to ignore what the HTTP method says. 

By contrast, a typical SOAP service keeps its method information in the entity-body 
and in a HTTP header. Example 1-8 is a Ruby script that searches the Web using 
Google's SOAP-based API. 

Example 1-8. Searching the Web with Google's search service 

#!/usr/bin/ruby -w 

# google-search.rb 
require 'soap/wsdlDriver' 

# Do a Google search and print out the title of each search result 
def print_page_titles(license_key, query) 

wsdl_uri = 'http://api.google.com/GoogleSearch.wsdl' 

driver = SOAP: :WSDLDriverFactory.new(wsdl_uri) .create_rpc_driver 

result_set = driver. doGoogleSearch(license_key, query, 0, 10, true, 1 ', 

false, '','','') 
result_set.resultElements.each { | result | puts result. title } 
end 

# Main program, 
if ARGV.size < 2 

puts "Usage: #{$0} [Google license key] [query]" 
exit 
end 

license_key, query = ARGV 
print_page_titles(license_key, query) 

« 

I While I was writing this book, Google announced that it was deprecat- 
*A ine its SOAP search service in favor of a RESTful, resource-oriented 
tK ' service (which, unfortunately, is encumbered by legal restrictions on use 
in a way the SOAP service isn't) . I haven't changed the example because 
Google's SOAP service still makes the best example I know of, and be- 
cause I don't expect you to actually run this program. I just want you 
to look at the code, and the SOAP and WSDL documents the code relies 
on. 

OK, that probably wasn't very informative, because the WSDL library hides most of 
the details. Here's what happens. When you call the doGoogleSearch method, the WSDL 
library makes a POST request to the "endpoint" of the Google SOAP service, located 
at the URI http://api.google.com/search/beta2. This single URI is the destination for 
every API call, and only POST requests are ever made to it. All of these details are in 



10 | Chapter 1: The Programmable Web and Its Inhabitants 



the WSDL file found at http://api.google.com/GoogleSearch.wsdl, which contains details 
like the definition of doGoogleSearch (Example 1-9). 

Example 1-9. Part of the WSDL description for Google's search service 
<operation name=" doGoogleSearch "> 

<input message="typens: doGoogleSearch "/> 

<output message="typens:doGoogleSearchResponse"/> 
</operation> 

Since the URI and the HTTP method never vary, the method information — that "do- 
GoogleSearch" — can't go in either place. Instead, it goes into the entity-body of the 
POST request. Example 1-10 shows what HTTP request you might make to do a search 
for REST. 

Example 1-10. A sample SOAP RPC call 

POST search/beta2 HTTP/l.l 
Host: api.google.com 
Content -Type: application/soap+xml 
SOAPAction : urn :GoogleSearchAction 

<?xml version="l.O" encoding="UTF-8"?> 

<soap: Envelope xmlns:soap="http://schemas. xmlsoap.org/soap/envelope/"> 
<soap:Body> 

<gs : doGoogleSearch xmlns : gs="urn : GoogleSearch"> 
<q>REST</q> 

</gs:doGoogleSearch> 
</soap:Body> 
</soap:Envelope> 

The method information is "doGoogleSearch . " That's the name of the XML tag inside 
the SOAP Envelope, it's the name of the operation in the WSDL file, and it's the name 
of the Ruby method in Example 1-8. It's also found in the value of the SOAPAction HTTP 
request header: some SOAP implementations look for it there instead of inside the 
entity-body. 

Let's bring things full circle by considering not the Google SOAP search API, but the 
Google search engine itself. To use your web browser to search Google's data set for 
REST, you'd send a GET request to httpu '/www. google. com/search?q=REST and get an 
HTML response back. The method information is kept in the HTTP method: you're 
GET ting a list of search results. 

Scoping Information 

The other big question web services answer differently is how the client tells the server 
which part of the data set to operate on. Given that the server understands that the 
client wants to (say) delete some data, how can it know which data the client wants to 
delete? Why should the server operate on this data instead of that data? 



Scoping Information | 11 



I call this information the scoping information. One obvious place to put it is in the URI 
path. That's what most web sites do. Think once again about a search engine URI like 
http://www.google.com/search?q=REST. There, the method information is "GET," and 
the scoping information is "/search?q=REST. " The client is trying to GET a list of search 
results about REST, as opposed to trying to GET something else: say, a list of search 
results about jellyfish (the scoping information for that would be "/search?q=jellyfish"), 
or the Google home page (that would be "/"). 

Many web services put scoping information in the path. Flickr's is one: most of the 
query variables in a Flickr API URI are scoping information. tags=penguin scopes the 
flickr. photos, search method so it only searches for photos tagged with "penguin." In 
a service where the method information defines a method in the programming language 
sense, the scoping information can be seen as a set of arguments to that method. You 
could reasonably expect to see flickr. photos. search(tags=penguin) as a line of code 
in some programming language. 

The alternative is to put the scoping information into the entity-body. A typical SOAP 
web service does it this way. Example 1-10 contains a q tag whose contents are the 
string "REST." That's the scoping information, nestled conveniently inside the 
doGoogleSearch tag that provides the method information. 

The service design determines what information is method information and what's 
scoping information. This is most obvious in cases like Flickr and Google, where the 
web site and the web service do the same thing but have different designs. These two 
URIs contain the same information: 

• http://fl.ickr. com/photos/tags/penguin 

• http://api.flickr.com/services/rest/?method=flickr.photos.search&tags=penguin 

In the first URI, the method information is "GET" and the scoping information is 
"photos tagged 'penguin.'" In the second URI, the method information is "do a photo 
search" and the scoping information is "penguin. " From a technical standpoint, there's 
no difference between the two: both of them use HTTP GET. The differences only 
become apparent at the level of architecture, when you take a step back and notice 
values for methodname like flickr . photos . delete, which take HTTP's GET method into 
places it wasn't meant to go. 

Another example: in the Google SOAP API, the fact that you're doing a search is method 
information (doGoogleSearch). The search query is scoping information (q). On the 
Google web site, both "search" and the value for "q" are scoping information. The 
method information is HTTP's standard GET. (If the Google SOAP API offered a 
method called doGoogleSearchForREST, it would be defining the method information so 
expansively that you'd need no scoping information to do a search for REST.) 



12 | Chapter 1: The Programmable Web and Its Inhabitants 



The Competing Architectures 

Now that I've identified the two main questions that web services answer differently, 
I can group web services by their answers to the questions. In my studies I've identified 
three common web service architectures: RESTful resource-oriented, RPC-style, and 
REST-RPC hybrid. I'll cover each in turn. 

RESTful, Resource-Oriented Architectures 

The main topic of this book is the web service architectures which can be considered 
RESTful: those which get a good score when judged on the criteria set forth in Roy 
Fielding's dissertation. Now, lots of architectures are technically RESTful, Abut I want 
to focus on the architectures that are best for web services. So when I talk about RESTful 
web services, I mean services that look like the Web. I'm calling this kind of service 
resource-oriented. In Chapter 3 I'll introduce the basic concepts of resource-oriented 
REST, in the context of a real web service: Amazon's Simple Storage Service. Starting 
in Chapter 5, I'll talk you through the defining characteristics of REST, and define a 
good architecture for RESTful web services: the Resource-Oriented Architecture. 

In RESTful architectures, the method information goes into the HTTP method. In Re- 
source-Oriented Architectures, the scoping information goes into the URL The com- 
bination is powerful. Given the first line of an HTTP request to a resource-oriented 
RESTful web service ("GET /reports/open-bugs HTTP/1.1"), you should understand 
basically what the client wants to do. The rest of the request is just details; indeed, you 
can make many requests using only one line of HTTP. If the HTTP method doesn't 
match the method information, the service isn't RESTful. If the scoping information 
isn't in the URI, the service isn't resource-oriented. These aren't the only requirements, 
but they're good rules of thumb. 

A few well-known examples of RESTful, resource-oriented web services include: 

• Services that expose the Atom Publishing Protocol (http://www.ietf.org/html.char 
ters/atompub-charter.html) and its variants such as GData (http://code.google.com/ 
apis/gdatal) 

• Amazon's Simple Storage Service (S3) (http://aws.amazon.com/s3) 

• Most of Yahool's web services (http://developer.yahoo.com/) 

• Most other read-only web services that don't use SOAP 

• Static web sites 

• Many web applications, especially read-only ones like search engines 



t More than you'd think. The Google SOAP API for web search technically has a RESTful architecture. So do 
many other read-only SOAP and XML-RPC services. But these are bad architectures for web services, because 
they look nothing like the Web. 



The Competing Architectures | 13 



Whenever I cover unRESTful architectures, as well as architectures that aren't resource- 
oriented, I do it with some ulterior motive. In this chapter, I want to put RESTful web 
services into perspective, against the larger backdrop of the programmable web. In 
Chapter 2, I'm widening the book's coverage of real web services, and showing that 
you can use the same client tools whether or not a service exactly fits my preferred 
architecture. In Chapter 10, I'm making an argument in a long-running debate about 
what the programmable web should look like. 

RPC-Style Architectures 

An RPC-style web service accepts an envelope full of data from its client, and sends a 
similar envelope back. The method and the scoping information are kept inside the 
envelope, or on stickers applied to the envelope. What kind of envelope is not important 
to my classification, but HTTP is a popular envelope format, since any web service 
worthy of the name must use HTTP anyway. SOAP is another popular envelope format 
(transmitting a SOAP document over HTTP puts the SOAP envelope inside an HTTP 
envelope). Every RPC-style service defines a brand new vocabulary. Computer pro- 
grams work this way as well: every time you write a program, you define functions with 
different names. By contrast, all RESTful web services share a standard vocabulary of 
HTTP methods. Every object in a RESTful service responds to the same basic interface. 

The XML-RPC protocol for web services is the most obvious example of the RPC ar- 
chitecture. XML-RPC is mostly a legacy protocol these days, but I'm going to start off 
with it because it's relatively simple and easy to explain. Example 1-11 shows a Ruby 
client for an XML-RPC service that lets you look up anything with a Universal Product 
Code. 

Example 1-11. An XML-RPC example: looking up a product by UPC 

#!/usr/bin/ruby -w 

# xmlrpc-upc.rb 

require 'xmlrpc/client' 
def find_product(upc) 

server = XMLRPC: : Client. new2( ' http://www.upcdatabase.com/rpc' ) 

begin 

response = server. call( 'lookupUPC' , upc) 
rescue XMtRPC: : FaultException => e 

puts "Error: " 

puts e.faultCode 

puts e.faultString 
end 
end 

puts find_product( "001441000055") [ ' description ' ] 

# "Trader Doe's Thai Rice Noodles" 

An XML-RPC service models a programming language like C. You call a function 
(lookupUPC) with some arguments ("001441000055") and get a return value back. The 



14 | Chapter 1: The Programmable Web and Its Inhabitants 



method data (the function name) and the scoping data (the arguments) are put inside 
an XML document. Example 1-12 gives a sample document. 

Example 1-12. An XML document describing an XML-RPC request 

<?xml version="l.O" ?> 
<methodCall> 

<methodName>lookupUPC</methodName> 
<params> 

<param><value><string>00144l000055</string></valuex/param> 
</params> 
</methodCall> 

This XML document is put into an envelope for transfer to the server. The envelope is 
an HTTP request with a method, URI, and headers (see Example 1-13). The XML 
document becomes the entity-body inside the HTTP envelope. 

Example 1-13. An HTTP envelope that contains an XML document which describes an XML-RPC 
request 

POST /rpc HTTP/l.l 

Host: www.upcdatabase.com 

User-Agent: XMLRPC: : Client (Ruby 1.8.4) 

Content-Type: text/xml; charset=utf-8 

Content- Length: 158 

Connection: keep-alive 

<?xml version="l.O" ?> 
<methodCall> 
<methodName>lookupUPC</methodName> 

</methodCall> 

The XML document changes depending on which method you're calling, but the HTTP 
envelope is always the same. No matter what you do with the UPC database service, 
the URI is always http://www.upcdatabase.com/rpc and the HTTP method is always 
POST. Simply put, an XML-RPC service ignores most features of HTTP. It exposes 
only one URI (the "endpoint"), and supports only one method on that URI (POST). 

Where a RESTful service would expose different URIs for different values of the scoping 
information, an RPC-style service typically exposes a URI for each "document pro- 
cessor": something that can open the envelopes and transform them into software 
commands. For purposes of comparison, Example 1-14 shows what that code might 
look like if the UPC database were a RESTful web service. 

Example 1-14. A hypothetical code sample: a RESTful UPC lookup service 
require 'open-uri' 

upc_data = open( ' http://www.upcdatabase.com/upc/0059849l' ) .read() 



The Competing Architectures | 15 



Here, the method information is contained in the HTTP method. The default HTTP 
method is GET, which is equivalent in this scenario to lookupUPC. The scoping infor- 
mation is contained in the URL The hypothetical service exposes an enormous number 
of URIs: one for every possible UPC. By contrast, the HTTP envelope is empty: an 
HTTP GET request contains no entity-body at all. 

For another example of a client for an RPC-style service, look back at Example 1-8. 
Google's SOAP search API is an RPC-style service that uses SOAP as its envelope 
format. 

A service that uses HTTP POST heavily or exclusively is probably an RPC-style service. 
Again, this isn't a sure sign, but it's a tip-off that the service isn't very interested in 
putting its method information in the HTTP method. An otherwise RESTful service 
that uses HTTP POST a lot tends to move toward a REST-RPC hybrid architecture. 

A few well-known examples of RPC-style web services: 

• All services that use XML-RPC 

• Just about every SOAP service (see the "Technologies on the Programmable 
Web" section later in this chapter for a defense of this controversial statement) 

• A few web applications (generally poorly designed ones) 

REST-RPC Hybrid Architectures 

This is a term I made up for describing web services that fit somewhere in between the 
RESTful web services and the purely RPC-style services. These services are often created 
by programmers who know a lot about real-world web applications, but not much 
about the theory of REST. 

Take another look at this URI used by the Flickr web service: http://www.flickr.com/ 
services/rest?api_key=xxx&method=flickr. photos. search&tags=penguin. Despite the 
"rest" in the URI, this was clearly designed as an RPC-style service, one that uses HTTP 
as its envelope format. It's got the scoping information ("photos tagged 'penguin'") in 
the URI, just like a RESTful resource-oriented service. But the method information 
("search for photos") also goes in the URI. In a RESTful service, the method information 
would go into the HTTP method (GET), and whatever was leftover would become 
scoping information. As it is, this service is simply using HTTP as an envelope format, 
sticking the method and scoping information wherever it pleases. This is an RPC-style 
service. Case closed. 

Except. ..look at Example 1-15. 

Example 1-15. A sample HTTP request to the Flickr web service 

GET services/rest ?api_key=xxx&method=f lickr . photos . search&tags=penguin HTTP/ 1 . 1 
Host: www.flickr.com 



16 | Chapter 1: The Programmable Web and Its Inhabitants 



That's the HTTP request a client makes when remotely calling this procedure. Now it 
looks like the method information is in the HTTP method. I'm sending a GET request 
to get something. What am I getting? A list of search results for photos tagged 
"penguin." What used to look like method information ("photoSearchO") now looks 
like scoping information ("photos/tag/penguin") . Now the web service looks RESTful. 

This optical illusion happens when an RPC-style service uses plain old HTTP as its 
envelope format, and when both the method and the scoping information happen to 
live in the URI portion of the HTTP request. If the HTTP method is GET, and the point 
of the web service request is to "get" information, it's hard to tell whether the method 
information is in the HTTP method or in the URI. Look at the HTTP requests that go 
across the wire and you see the requests you'd see for a RESTful web service. They may 
contain elements like "method=flickr. photos. search" but that could be interpreted as 
scoping information, the way "photos/" and "search/" are scoping information. These 
RPC-style services have elements of RESTful web services, more or less by accident. 
They're only using HTTP as a convenient envelope format, but they're using it in a way 
that overlaps with what a RESTful service might do. 

Many read-only web services qualify as entirely RESTful and resource-oriented, even 
though they were designed in the RPC style! But if the service allows clients to write to 
the data set, there will be times when the client uses an HTTP method that doesn't 
match up with the true method information. This keeps the service from being as 
RESTful as it could be. Services like these are the ones I consider to be REST-RPC 
hybrids. 

Here's one example. The Flickr web API asks clients to use HTTP GET even when they 
want to modify the data set. To delete a photo you make a GET request to a URI that 
includes method=f lickr . photos . delete. That's just not what GET is for, as I'll show in 
"Split the Data Set into Resources [115]. The Flickr web API is a REST-RPC hybrid: 
RESTful when the client is retrieving data through GET, RPC-style when the client is 
modifying the data set. 

A few well-known examples of REST-RPC hybrid services include: 

• The del.icio.us API 

• The "REST" Flickr web API 

• Many other allegedly RESTful web services 

• Most web applications 

From a design standpoint, I don't think anybody sets out to to design a service as a 
REST-RPC hybrid. Because of the way HTTP works, any RPC-style service that uses 
plain HTTP and exposes multiple URIs tends to end up either RESTful or hybrid. Many 
programmers design web services exactly as they'd design web applications, and end 
up with hybrid services. 

The existence of hybrid architectures has caused a lot of confusion. The style comes 
naturally to people who've designed web applications, and it's often claimed that hy- 



The Competing Architectures | 17 



brid architectures are RESTful: after all, they work "the same way" as the human web. 
A lot of time has been spent trying to distinguish RESTful web services from these 
mysterious others. My classification of the "others" as REST-RPC hybrids is just the 
latest in a long line of neologisms. I think this particular neologism is the most accurate 
and useful way to look at these common but baffling services. If you've encountered 
other ways of describing them ("HTTP+POX" is the most popular at the time of writ- 
ing), you might want read on, where I explain those other phrases in terms of what I'm 
saying in this book. 

The Human Web Is on the Programmable Web 

In the previous sections I claimed that all static web sites are RESTful. I claimed that 
web applications fall into one of the three categories, the majority being REST-RPC 
hybrids. Since the human web is made entirely of static web sites and web applications, 
this means that the entire human web is also on the programmable web! By now this 
should not be surprising to you. A web browser is a software program that makes HTTP 
requests and processes the responses somehow (by showing them to a human). That's 
exactly what a web service client is. If it's on the Web, it's a web service. 

My goal in this book is not to make the programmable web bigger. That's almost im- 
possible: the programmable web already encompasses nearly everything with an HTTP 
interface. My goal is to help make the programmable web better: more uniform, better- 
structured, and using the features of HTTP to greatest advantage. 

Technologies on the Programmable Web 

I've classified web services by their underlying architectures, distinguishing the fish 
from the whales. Now I can examine the technologies they use, without confusing 
technology and architecture. 

HTTP 

All web services use HTTP, but they use it in different ways. A request to a RESTful 
web service puts the method information in the HTTP method and the scoping infor- 
mation in the URI. RPC-style web services tend to ignore the HTTP method, looking 
for method and scoping information in the URI, HTTP headers, or entity-body. Some 
RPC-style web services use HTTP as an envelope containing a document, and others 
only use it as an unlabelled envelope containing another envelope. 

URI 

Again, all web services use URIs, but in different ways. What I'm about to say is a 
generalization, but a fairly accurate one. A RESTful, resource-oriented service exposes 
a URI for every piece of data the client might want to operate on. A REST-RPC hybrid 



18 | Chapter 1: The Programmable Web and Its Inhabitants 



exposes a URI for every operation the client might perform: one URI to fetch a piece 
of data, a different URI to delete that same data. An RPC-style service exposes one URI 
for every processes capable of handling Remote Procedure Calls (RPC) . There's usually 
only one such URI: the service "endpoint." 

XML-RPC 

A few, mostly legacy, web services use XML-RPC on top of HTTP. XML-RPC is a data 
structure format for representing function calls and their return values. As the name 
implies, it's explicitly designed to use an RPC style. 

SOAP 

Lots of web services use SOAP on top of HTTP. SOAP is an envelope format, like HTTP, 
but it's an XML-based envelope format. 

Now I'm going to say something controversial. To a first approximation, every current 
web service that uses SOAP also has an RPC architecture. This is controversial because 
many SOAP programmers think the RPC architecture is declasse and prefer to call their 
services "message-oriented" or "document-oriented" services. 

Well, all web services are message-oriented, because HTTP itself is message-oriented. 
An HTTP request is just a message: an envelope with a document inside. The question 
is what that document says. SOAP -based services ask the client to stick a second en- 
velope (a SOAP document) inside the HTTP envelope. Again, the real question is what 
it says inside the envelope. A SOAP envelope can contain any XML data, just as an 
HTTP envelope can contain any data in its entity-body. But in every existing SOAP 
service, the SOAP envelope contains a description of an RPC call in a format similar to 
that of XML-RPC. 

There are various ways of shuffling this RPC description around and giving it different 
labels — "document/literal" or "wrapped/literal" — but any way you slice it, you have a 
service with a large vocabulary of method information, a service that looks for scoping 
information inside the document rather than on the envelope. These are defining fea- 
tures of the RPC architecture. 

I emphasize that this is not a fact about SOAP, just a fact about how it's currently used. 
SOAP, like HTTP, is just a way of putting data in an envelope. Right now, though, the 
only data that ever gets put in that envelope is XML-RPC-esque data about how to call 
a remote function, or what's the return value from such a function. I argue this point 
in more detail in Chapter 10. 

WS-* 

These standards define special XML "stickers" for the SOAP envelope. The stickers are 
analagous to HTTP headers. 



Technologies on the Programmable Web | 1 9 



WSDL 



The Web Service Description Language (WSDL) is an XML vocabulary used to describe 
SOAP-based web services. A client can load a WSDL file and know exactly which RPC- 
style methods it can call, what arguments those methods expect, and which data types 
they return. Nearly every SOAP service in existence exposes a WSDL file, and most 
SOAP services would be very difficult to use without their WSDL files serving as guides. 
As I discuss in Chapter 10, WSDL bears more responsiblity than any other technology 
for maintaining SOAP's association with the RPC style. 

WADL 

The Web Application Description Language (WADL) is an XML vocabulary used to 
describe RESTful web services. As with WSDL, a generic client can load a WADL file 
and be immediately equipped to access the full functionality of the corresponding web 
service. I discuss WADL in Chapter 9. 

Since RESTful services have simpler interfaces, WADL is not nearly as necessary to 
these services as WSDL is to RPC-style SOAP services. This is a good thing, since as of 
the time of writing there are few real web services providing official WADL files. Ya- 
hool's web search service is one that does. 

Leftover Terminology 

Believe it not, there are some common terms used in discussions of REST that I haven't 
mentioned yet. I haven't mentioned them because I think they're inaccurate or entirely 
outside the scope of this book. But I owe you explanations of why I think this, so you 
can decide whether or not you agree. Feel free to skip this section if you haven't heard 
these terms. 

Service-Oriented Architecture 

This is a big industry buzzword. I'm not going to dwell on it for two reasons. First, 
the term is not very well defined. Second, to the extent that it is defined, it means 
something like: "a software architecture based on the production and consumption 
of web services. " In this book I talk about the design of individual services. A book 
on service-oriented architecture should work on a slightly higher level, showing 
how to use services as software components, how to integrate them into a coherent 
whole. I don't cover that sort of thing in this book. 

SOAP as a competitor to REST 

If you get involved with web service debates you'll hear this one a lot. You won't 
hear it here because it gives the wrong impression. The primary competitors to 
RESTful architectures are RPC architectures, not specific technologies like SOAP. 
It is true that basically every SOAP service that now exists has an RPC architecture, 
but SOAP is just a way of putting a document in an envelope with stickers on it, 



20 | Chapter 1: The Programmable Web and Its Inhabitants 



like HTTP. SOAP is tied to the RPC architecture mainly by historical contingency 
and the current generation of automated tools. 

There is a real tension here, but it's not one I'll cover much in this book. Roughly 
speaking, it's the tension between services that put their documents in a SOAP 
envelope and then an HTTP envelope; and services that only use the HTTP enve- 
lope. 

HTTP+POX 

Stands for HTTP plus Plain Old XML. This term covers roughly those services I 
call REST-RPC hybrid services. They overlap with RESTful designs, especially 
when it comes to retrieving data, but their basic architecture is RPC-oriented. 

I don't like this term because Plain Old XML is inaccurate. The interesting thing 
about these services is not that they produce plain old XML documents (as opposed 
to XML documents wrapped in SOAP envelopes). Some of these services don't 
serve XML at all: they serve JSON, plain text, or binary files. No, the interesting 
thing about these services is their RPC architecture. That's what puts them in op- 
position to REST. 

STREST 

Means Service-Trampled REST. This is another term for REST-RPC hybrid archi- 
tectures. It's more accurate than HTTP+POX since it conveys the notion of a 
RESTful architecture taken over by something else: in this case, the RPC style. 

This is a cute acronym but I don't like it, because it buys into a myth that the only 
true web services are RPC-style services. After all, the service that trampled your 
REST was an RPC service. If you think that REST services are real services, it 
doesn't make sense to cry "Help! I had some REST but then this Service got into 
it!" RPC-Trampled REST would be more accurate, but that's a lousy acronym. 

High and low REST 

Yet another way of distinguishing between truly RESTful services and the ones I 
call REST-RPC hybrids. High REST services are just those that adhere closely to 
the Fielding dissertation. Among other things, they put method information in the 
HTTP method and scoping information in the URI. Low REST services are pre- 
sumed to have deviated. Since low REST services tend to deviate from orthodoxy 
in a particular direction (toward the RPC style), I prefer a more specific 
terminology. 



Leftover Terminology | 21 



CHAPTER 2 



Writing Web Service Clients 



Web Services Are Web Sites 

In Chapter 1 1 showed some quick examples of clients for existing, public web services. 
Some of the services had resource-oriented RESTful architectures, some had RPC-style 
architectures, and some were hybrids. Most of the time, I accessed these services 
through wrapper libraries instead of making the HTTP requests myself. 

You can't always rely on the existence of a convenient wrapper library for your favorite 
web service, especially if you wrote the web service yourself. Fortunately, it's easy to 
write programs that work directly with HTTP requests and responses. In this chapter 
I show how to write clients for RESTful and hybrid architecture services, in a variety 
of programming languages. 

Example 2-1 is a bare HTTP client for a RESTful web service: Yahool's web search. 
You might compare it to Example 1-8, the client from the previous chapter that runs 
against the RPC-style SOAP interface to Google's web search. 

Example 2-1. Searching the Web with Yahool's web service 

#!/usr/bin/ruby 
# yahoo-web-search. rb 
require 'open-uri' 
require 'rexml/document' 
require 'cgi' 

BASEJJRI = ' http : //api . search . yahoo. com/WebSearchService/Vl/webSearch ' 

def print_page_titles(term) 

# Fetch a resource: an XML document full of search results, 
term = CGI: :escape(term) 

xml = open (BASEJJRI + "?appid=restbook&query=#{term}").read 

# Parse the XML document into a data structure, 
document = REXML: : Document. new(xml) 

# Use XPath to find the interesting parts of the data structure. 
REXML: :XPath.each(document, '/ResultSet/Result/Title/[] ' ) do | title | 



23 



puts title 
end 
end 

(puts "Usage: #{$0} [search term]"; exit) if ARCV. empty? 
print_page_titles(ARGV. join( ' ' )) 

This "web service" code looks just like generic HTTP client code. It uses Ruby's stand- 
ard open-uri library to make an HTTP request and Ruby's standard REXML library to 
parse the output. I'd use the same tools to fetch and process a web page. These two 
URIs: 

• http://api.search.yahoo.com/WebSearchService/Vl /webSearch?appid=rest 
book&query=jellyfish 

• http://search.yahoo.com/search?p=jellyfish 

point to different forms of the same thing: "a list of search results for the query 'jelly- 
fish.' " One URI serves HTML and is intended for use by web browsers; the other serves 
XML and is intended for use by automated clients. 



XPath Exposition 

Reading from right to left, the expression /ResultSet/Result/Title/[] means: 

Find the direct children [] 

of every Title tag Title/ 

that's the direct child of a Result tag Result/ 

that's the direct child of the ResultSet tag ResultSet/ 

at the root of the document. / 

If you look at the XML files served by the Yahoo! search service, you'll see a Result 
Set tag that contains Result tags, each of which contains a Title tag. The contents of 
those tags are what I'm after in Example 2-1. 



There is no magic dust that makes an HTTP request a web service request. You can 
make requests to a RESTful or hybrid web service using nothing but your programming 
language's HTTP client library. You can process the results with a standard XML pars- 
er. Every web service request involves the same three steps: 

1. Come up with the data that will go into the HTTP request: the HTTP method, the 
URI, any HTTP headers, and (for requests using the PUT or POST method) any 
document that needs to go in the request's entity-body. 

2. Format the data as an HTTP request, and send it to the appropriate HTTP server. 



24 | Chapter 2: Writing Web Service Clients 



3. Parse the response data — the response code, any headers, and any entity-body — 
into the data structures the rest of your program needs. 

In this chapter I show how different programming languages and libraries implement 
this three-step process. 

Wrappers, WADL, and ActiveResource 

Although a web service request is just an HTTP request, any given web service has a 
logic and a structure that is missing from the World Wide Web as a whole. If you follow 
the three-step algorithm every time you make a web service request, your code will be 
a mess and you'll never take advantage of that underlying structure. 

Instead, as a smart programmer you'll quickly notice the patterns underlying your re- 
quests to a given service, and write wrapper methods that abstract away the details of 
HTTP access. The print_page_titles method defined in Example 2-1 is a primitive 
wrapper. As a web service gets popular, its users release polished wrapper libraries in 
various languages. Some service providers offer official wrappers: Amazon gives away 
clients in five different languages for its RESTful S3 service. That hasn't stopped outside 
programmers from writing their own S3 client libraries, like jbucket and s3sh. 

Wrappers make service programming easy, because the API of a wrapper library is 
tailored to one particular service. You don't have to think about HTTP at all. The 
downside is that each wrapper is slightly different: learning one wrapper doesn't pre- 
pare you for the next one. 

This is a little disappointing. After all, these services are just variations on the three- 
step algorithm for making HTTP requests. Shouldn't there be some way of abstracting 
out the differences between services, some library that can act as a wrapper for the 
entire space of RESTful and hybrid services? 

This is the problem of service description. We need a language with a vocabulary that 
can describe the variety of RESTful and hybrid services. A document written in this 
language could script a generic web service client, making it act like a custom-written 
wrapper. The SOAP RPC community has united around WSDL as its service descrip- 
tion language. The REST community has yet to unite around a description language, 
so in this book I do my bit to promote WADL as a resource-oriented alternative to 
WSDL. I think it's the simplest and most elegant solution that solves the whole prob- 
lem. I show a simple WADL client in this chapter and it is covered in detail in the 
"WADL" section. 

There's also a generic client called ActiveResource, still in development. ActiveRe- 
source makes it easy to write clients for many kinds of web services written with the 
Ruby on Rails framework. I cover ActiveResource at the end of Chapter 3. 



Web Services Are Web Sites | 25 



del.icio.us / leonardr / restbook 



your bookmarks | your network j subscriptions j links for you | post 



i view 



.11, po 



mmendations 



dations 



Joe Gregorlo | BitWorkjng | Projects f Atom Publishing Protocol Test 

Client emt / oeiea 

lu stwhat the doctor ordered 
Ihosetf' — H If 1 1 days ago 

thisURI a llnifnrm Hnqnnrrn Mamp i\ IRNI |Mamnt:narfi fnr Tan Metadata »dtt 

Social features 



Figure 2-1. del.icio.us screenshot 

del.icio.us: The Sample Application 

In this chapter I walk through the life cycle of a web service request from the client's 
point of view. Though most of this book's code examples are written in Ruby, in this 
chapter I show code written in a variety of programming languages. My example 
throughout this chapter is the web service provided by the social bookmarking web site 
del.icio.us (http://del.icio.us/) . You can read a prose description of this web service at 
http://del.icio.us/help/api/. 



One of my ! 
bookmarked - 

mis 



If you're not familiar with del.icio.us, here's a brief digressionary intro- 
duction, del.icio.us is a web site that works like your web browser's 
* bookmark feature, but it's public and better-organized (see Fig- 
ure 2-1). When you save a link to del.icio.us, it's associated with your 
account so you can find it later. You can also share your bookmarks 
with others. 

You can associate short strings, called tags, with a URL Tags are versatile 
little suckers. They make it easy for you to find a URI later, they make 
it possible to group URIs together, and when multiple people tag the 
same URI, they create a machine-readable vocabulary for that URI. 



The del.icio.us web service gives you programmatic access to your bookmarks. You 
can write programs that bookmark URIs, convert your browser bookmarks to 
del.icio.us bookmarks, or fetch the URIs you've bookmarked in the past. The best way 
to visualize the del.icio.us web service is to use the human-oriented web site for a while. 
There's no fundamental difference between the del.icio.us web site and the del.icio.us 
web service, but there are variations: 



26 | Chapter 2: Writing Web Service Clients 



• The web site is rooted at http://del.icio.us/ and the web service is rooted at https:// 
api.del.icio.us/vl/. The web site communicates with clients through HTTP, the web 
service uses secure HTTPS. 

• The web site and the web service expose different URI structures. To get your recent 
bookmarks from the web site, you fetch https: //del. icio. us/ {your-username}. To 
get your recent bookmarks from the web service, you fetch https://api.del.icio.us/ 
vl/posts/recent. 

• The web site serves HTML documents, and the web service serves XML docu- 
ments. The formats are different, but they contain the same data. 

• The web site lets you see a lot of information without logging in or even having an 
account. The web service makes you authenticate for every request. 

• Both offer features for personal bookmark management, but the web site also has 
social features. On the web site, you can see lists of URIs other people have book- 
marked, lists of people who have bookmarked a particular URI, lists of URIs tagged 
with a certain tag, and lists of popular bookmarks. The web service only lets you 
see your own bookmarks. 

These variations are important but they don't make the web service a different kind of 
thing from the web site. The web service is a stripped-down web site that uses HTTPS 
and serves funny-looking documents. (You can flip this around and look at the web 
site as a more functional web service, though the del. icio. us administrators discourage 
this viewpoint.) This is a theme I'm coming back to again and again: web services should 
work under the same rules as web sites. 

Aside from its similarity to a web site, the del. icio. us web service does not have a very 
RESTful design. The programmers have laid out the service URIs in a way that suggests 
an RPC-style rather than a resource-oriented design. All requests to the del. icio. us web 
service use the HTTP GET method: the real method information goes into the URI and 
might conflict with "GET". A couple sample URIs should illustrate this point: consider 
https://api.del.icio.us/vl/posts/add and https://api.del.icio.us/vl/tags/rename. Though 
there's no explicit methodName variable, the del. icio. us API is just like the Flickr API I 
covered in Chapter 1. The method information ("add" and "rename") is kept in the 
URIs, not in the HTTP method. 

So why have I chosen del. icio. us for the sample clients in this chapter? Three reasons. 
First, del. icio. us is an easy application to understand, and its web service is popular 
and easy to use. 

Second, I want to make it clear that what I say in the coming chapters is prescriptive, 
not descriptive. When you implement a web service, following the constraints of REST 
will give your clients a nice, usable web service that acts like the web. But when you 
implement a web service client, you have to work with the service as it is. The only 
alternatives are to lobby for a change or boycott the service. If a web service designer 
has never heard of REST, or thinks that hybrid services are "RESTful," there's little you 
can do about it. Most existing services are hybrids or full-blown RPC services. A snooty 



del.icio.us: The Sample Application | 27 



client that can feed only on the purest of REST services isn't very useful, and won't be 
for the forseeable future. Servers should be idealistic; clients must be pragmatic. This 
is a variant of Postel's Law: "Be conservative in what you do; be liberal in which you 
accept from others." 

Third, in Chapter 7 I present a bookmark-tracking web service that's similar to 
del.icio.us but designed on RESTful principles. I want to introduce the social book- 
marking domain to you now, so you'll be thinking about it as I introduce the principles 
of REST and my Resource-Oriented Architecture. In Chapter 7, when I design and 
implement a RESTful interface to del.icio.us-like functionality, you'll see the difference. 

What the Sample Clients Do 

In the sections that follow, I show you simple del.icio.us clients in a variety of pro- 
gramming languages. All of these clients do exactly the same thing, and it's worth 
spelling out what that is. First, they open up a TCP/IP socket connection to port 443 
(the standard HTTPS port) on the server at api . del . icio .us. Then they send something 
like the HTTP request in Example 2-2. The del.icio.us web service sends back some- 
thing like the HTTP response in Example 2-3, then closes the socket connection. Like 
all HTTP responses, this one has three parts: a status code, a set of headers, and an 
entity-body. In this case, the entity-body is an XML document. 

Example 2-2. A possible request to the del.icio.us web service 

GET /vl/posts/recent HTTP/l.l 
Host: api. del. icio. us 

Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ= 

Example 2-3. A possible response from the del.icio.us web service 
200 OK 

Content-Type: text/xml 

Date: Sun, 29 Oct 2006 15:09:36 GMT 

Connection: close 

<?xml version='l.0' standalone='yes' ?> 
<posts tag="" user="username"> 
<post href =" http://www.foo. com/" description="foo" extended="" 

hash="l4d59bdc067e3clf8f792f5l010ae5ac" tag="foo" 

time="2006-10-29T02:56:12Z" /> 

<post href="http://amphibians.com/" description="Amphibian Mania" 
extended="" hash="688b7b2f224lbc54aOb267b69f438805" tag="frogs toads" 
time="2006-10-28T02:55:53Z" /> 
</posts> 

The clients I write are only interested in the entity-body part. Specifically, they're only 
interested in the href and description attributes of the post tags. They'll parse the XML 
document into a data structure and use the XPath expression /posts/post to iterate 
over the post tags. They'll print to standard output the href and description attribute 
of every del.icio.us bookmark: 



28 | Chapter 2: Writing Web Service Clients 



-foo: http://www.foo.com/ 

Amphibian Mania: http://amphibians.com/ 



XPath Exposition 

Reading from right to left, the XPath expression /posts/post means: 

Find every post tag post 
that's the direct child of the posts tag posts/ 
attherootofthedocument. / 



Making the Request: HTTP Libraries 

Every modern programming language has one or more libraries for making HTTP re- 
quests. Not all of these libraries are equally useful, though. To build a fully general web 
service client you need an HTTP library with these features: 

• It must support HTTPS and SSL certificate validation. Web services, like web sites, 
use HTTPS to secure communication with their clients. Many web services 
(del.icio.us is one example) won't accept plain HTTP requests at all. A library's 
HTTPS support often depends on the presense of an external SSL library written 
in C. 

• It must support at least the five main HTTP methods: GET, HEAD, POST, PUT, 
and DELETE. Some libraries support only GET and POST. Others are designed 
for simplicity and support only GET. 

You can get pretty far with a client that only supports GET and POST: HTML 
forms support only those two methods, so the entire human web is open to you. 
You can even do all right with just GET, because many web services (among them 
del.icio.us and Flickr) use GET even where they shouldn't. But if you're choosing 
a library for all your web service clients, or writing a general client like a WADL 
client, you need a library that supports all five methods. Additional methods like 
OPTIONS and TRACE, and WebDAV extensions like MOVE, are a bonus. 

• It must allow the programmer to customize the data sent as the entity-body of a 
PUT or POST request. 

• It must allow the programmer to customize a request's HTTP headers. 

• It must give the programmer access to the response code and headers of an HTTP 
response; not just access to the entity-body. 

• It must be able to communicate through an HTTP proxy. The average programmer 
may not think about this, but many HTTP clients in corporate environments can 



Making the Request: HTTP Libraries | 29 



only work through a proxy. Intermediaries like HTTP proxies are also a standard 
part of the REST meta-architecture, though not one I'll be covering in much detail. 

Optional Features 

There are also some features of an HTTP library that make life easier as you write clients 
for RESTful and hybrid services. These features mostly boil down to knowledge about 
HTTP headers, so they're technically optional. You can implement them yourself so 
long as your library gives you access to request and response HTTP headers. The ad- 
vantage of library support is that you don't have to worry about the details. 

• An HTTP library should automatically request data in compressed form to save 
bandwidth, and transparently decompress the data it receives. The HTTP request 
header here is Accept- Encoding, and the response header is Encoding. I discuss these 
in more detail in Chapter 8. 

• It should automatically cache the responses to your requests. The second time you 
request a URI, it should return an item from the cache if the object on the server 
hasn't changed. The HTTP headers here are ETag and If-Modified-Since for the 
request, and Etag and Last-Modified for the response. These, too, I discuss in 
Chapter 8. 

• It should transparently support the most common forms of HTTP authentica- 
tion: Basic, Digest, and WSSE. It's useful to support custom, company-specific 
authentication methods such as Amazon's, or to have plug-ins that support them. 

The request header is Authorization and the response header (the one that de- 
mands authentication) is WWW-Authenticate. I cover the standard HTTP authenti- 
cation methods, plus WSSE, in Chapter 8. 1 cover Amazon's custom authentication 
method in Chapter 3. 

• It should be able to transparently follow HTTP redirects, while avoiding infinite 
redirects and redirect loops. This should be an optional convenience for the user, 
rather than something that happens on every single redirect. A web service may 
reasonably send a status code of 303 ("See Other") without implying that the client 
should go fetch that other URI right now! 

• It should be able to parse and create HTTP cookie strings, rather than forcing the 
programmer to manually set the Cookie header. This is not very important for 
RESTful services, which shun cookies, but it's very important if you want to use 
the human web. 

When you're writing code against a specific service, you may be able to do without 
some or all of these features. Ruby's standard open-uri library only supports GET re- 
quests. If you're writing a client for del.icio.us, there's no problem, since that web 
service expects only GET requests. But try to use open-uri with Amazon S3 (which uses 
GET, HEAD, PUT, and DELETE), and you'll quickly run into a wall. In the next sec- 



30 | Chapter 2: Writing Web Service Clients 



tions I recommend good HTTP client libraries for some popular programming 
languages. 

Ruby: rest-open-uri and net/http 

Ruby comes with two HTTP client libraries, open-uri and the lower-level net/http. 
Either can make HTTPS requests if you've got the net/https extension installed. Win- 
dows installations of Ruby should be able to make HTTPS requests out of the box. If 
you're not on Windows, you may have to install net/https separately.* 

The open-uri library has a simple and elegant interface that lets you treat URIs as file- 
names. To read a web page, you simply open its URI and read data from the "filehandle." 
You can pass in a hash to open containing custom HTTP headers and open-specific 
keyword arguments. This lets you set up a proxy, or specify authentication information. 

Unfortunately, right now open-uri only supports one HTTP method: GET. That's why 
I've made some minor modifications to open-uri and made the result available as the 
rest-open-uri Ruby gem.t I've added two keyword arguments to open: : method, which 
lets you customize the HTTP method, and : body, which lets you send data in the entity- 
body. 

Example 2-4 is an implementation of the standard del.icio.us example using the open- 
uri library (rest-open-uri works the same way). This code parses the response docu- 
ment using the REXML: : Document parser, which you've seen before. 

Example 2-4. A Ruby client using open-uri 

#!/usr/bin/ruby -w 

# delicious-open-uri.rb 

require 'rubygems' 
require 'open-uri' 
require 'rexml/document' 

# Fetches a del.icio.us user's recent bookmarks, and prints each one. 
def print_my_recent_bookmarks(username, password) 

# Make the HTTPS request. 

response = open( ' https ://api.del.icio.us/vl/posts/recent ' , 

:http_basic_authentication => [username, password]) 

# Read the response entity-body as an XML document, 
xml = response. read 



On Debian GNU/Linux and Debian-derived systems like Ubuntu, the package name is libopenssl-ruby. If 
your packaging system doesn't include net/https, you'll have to download it from http://www.nongnu.org/ 
rubypki/ and install it by hand. 

t For more information on Ruby gems, see http://rubygems.org/. Once you have the gem program installed, you 
can install rest-open-uri with the command gem install rest-open-uri. Hopefully my modifications to 
open-uri will one day make it into the core Ruby code, and the rest-open-uri gem will become redundant. 



Making the Request: HTTP Libraries | 31 



# Turn the document into a data structure, 
document = REXML: : Document. new(xml) 

# For each bookmark... 

REXML: :XPath.each(document, "/posts/post") do |e| 
# Print the bookmark's description and URI 
puts "#{e. attributes [' description']}: #{e.attributes[ ' href ' ] }" 
end 
end 

# Main program 

username, password = ARCV 

unless username and password 

puts "Usage: #{$0} [username] [password]" 

exit 
end 

print_my_recent_bookmarks (username, password) 

I mentioned earlier that Ruby's stock open-uri can only make HTTP GET requests. For 
many purposes, GET is enough, but if you want to write a Ruby client for a fully RESTful 
service like Amazon's S3, you'll either need to use rest-open-uri, or turn to Ruby's 
low-level HTTP library: net/http. 

This built-in library provides the Net : : HTTP class, which has several methods for making 
HTTP requests (see Table 2-1). You can build a complete HTTP client out of this class, 
using nothing more than the Ruby standard library. In fact, open-uri and rest-open- 
uri are based on Net: :HTTP. Those libraries only exist because Net: :HTTP provides no 
simple, easy-to-use interface that supports all the features a REST client needs (proxies, 
HTTPS, headers, and so on). That's why I recommend you use rest-open-uri. 

Table 2-1. HTTP feature matrix for Ruby HTTP client libraries 



HTTPS 


Yes (assuming the net/https library is installed) 






HTTP verbs 


GET 


All 


All 


Custom data 


No 


Yes 


Yes 


Custom headers 


Yes 






Proxies 


Yes 






Compression 


No 






Caching 


No 






Auth methods 


Basic 






Cookies 


No 






Redirects 


Yes 


Yes 


No 



32 | Chapter 2: Writing Web Service Clients 



Python: httplib2 

The Python standard library comes with two HTTP clients: urllib2, which has a file- 
like interface like Ruby's open-uri; and httplib, which works more like Ruby's 
Net: :HTTP. Both offer transparent support for HTTPS, assuming your copy of Python 
was compiled with SSL support. There's also an excellent third-party library, Joe Gre- 
gorio's httplib2 (http://bitworking.org/projects/httplib2/), which is the one I recom- 
mend in general. httplib2 is an excellent piece of software, supporting nearly every 
feature on my wish list — most notably, transparent caching. Table 2-2 lists the features 
available in each library. 



Table 2-2. HTTP feature matrix for Python HTTP client libraries 



HTTPS 


Yes (assuming Python was compiled with SSL support) 






HTTP verbs 


GET, POST 


All 


All 


Custom data 


Yes 






Custom headers 


Yes 






Proxies 


Yes 


No 


No 


Compression 


No 


No 


Yes 


Caching 


No 


No 


Yes 


Auth methods 


Basic, Digest 


None 


Basic, Digest, WSSE, Goo- 
gle 


Cookies 


Yes (Use urllib2. build opener (HTTPCookieProces 
sor)) 


No 


No 


Redirects 


Yes 


No 


Yes 



Example 2-5 is a del.icio.us client that uses httplib2. It uses the ElementTree library 
to parse the del.icio.us XML. 

Example 2-5. A del.icio.us client in Python 

#!/usr/bin/python2.5 

# delicious-httplib2.py 
import sys 

from xml.etree import ElementTree 
import httplib2 

# Fetches a del.icio.us user's recent bookmarks, and prints each one. 
def print_my_recent_bookmarks(username, password): 

client = httplib2.Http(". cache") 

client. add_credentials(username, password) 

# Make the HTTP request, and fetch the response and the entity-body, 
response, xml = client. request( ' https://api.del.icio.us/vl/posts/recent ' ) 

# Turn the XML entity-body into a data structure. 



Making the Request: HTTP Libraries | 33 



doc = ElementTree.fromstring(xml) 

# Print information about every bookmark, 
for post in doc. f indall( ' post ') : 

print "%s: %s" % (post. attrib[ 'description' ] , post .attrib[ ' href ' ]) 

# Main program 

if len(sys.argv) != 3: 

print "Usage: %s [username] [password]" % sys.argv[o] 

sys.exit() 

username, password = sys.argv[l: ] 
print_my_recent_bookmarks (username, password) 

Java: HttpClient 

The Java standard library comes with an HTTP client, java.net.HttpURLConnection. 
You can get an instance by calling open on a java.net. URL object. Though it supports 
most of the basic features of HTTP, programming to its API is very difficult. The Apache 
Jakarta project has a competing client called HttpClient (http://jakarta.apache.org/com 
mons/httpclientf) , which has a better design. There's also Restlet (http://www.rest 
let.org/). I cover Restlet as a server library in Chapter 12, but it's also an HTTP client 
library. The class org. restlet. Client makes it easy to make simple HTTP requests, 
and the class org. restlet. data. Request hides the HttpURLConnection programming 
necessary to make more complex requests. Table 2-3 lists the features available in each 
library. 

Table 2-3. HTTP feature matrix for Java HTTP client libraries. 



HTTPS 


Yes 






HTTP verbs 


All 






Custom data 


Yes 






Custom headers 


Yes 






Proxies 


Yes 






Compression 


No 


No 


Yes 


Caching 


Yes 


No 


Yes 


Auth methods 


Basic, Digest, NTLM 




Basic, Amazon 


Cookies 


Yes 






Redirects 


Yes 




IT 



Example 2-6 is a Java client for del.icio.us that uses HttpClient. It works in Java 1.5 
and up, and it'll work in previous versions if you install the Xerces parser (see "Java: 
javax.xml, Xerces, or XMLPull" later in this chapter). 



34 | Chapter 2: Writing Web Service Clients 



Example 2-6. A del.icio.us client in Java 

II DeliciousApp. java 
import java. io.*; 

import org .apache . commons . httpclient . * ; 

import org . apache. commons . httpclient . auth . AuthScope; 

import org .apache . commons . httpclient . methods . GetMethod ; 

import org.w3c.dom.*; 
import org.xml.sax.SAXException; 
import javax.xml. parsers.*; 
import javax.xml. xpath.*; 

/** 

* A command-line application that fetches bookmarks from del.icio.us 

* and prints them to standard output. 

*/ 

public class DeliciousApp 

{ 

public static void main(String[ ] args) 
throws HttpException, IOException, ParserConfigurationException, 
SAXException, XPathExpressionException 

{ 

if (args. length != 2) 
{ 

System. out. println("Usage: java -classpath [CLASSPATH] " 

+ "DeliciousApp [USERNAME] [PASSWORD]"); 

System. out. println(" [CLASSPATH] - Must contain commons-codec, " + 
"commons-logging, and commons-httpclient"); 

System. out. println(" [USERNAME] - Your del.icio.us username"); 

System. out. println(" [PASSWORD] - Your del.icio.us password"); 

System.out.printlnQ; 

System. exit(-l); 

} 

// Set the authentication credentials. 

Credentials creds = new UsernamePasswordCredentials(args[o] , args[l]); 

HttpClient client = new HttpClientQ; 

client. getStateQ .setCredentials (AuthScope. ANY, creds); 

// Make the HTTP request. 

String url = "https://api.del.icio.us/vl/posts/recent"; 
GetMethod method = new GetMethod (url); 
client . executeMethod (method ) ; 

InputStream responseBody = method. getResponseBodyAsStreamQ; 

// Turn the response entity-body into an XML document. 
DocumentBuilderFactory docBuilderFactory = 

DocumentBuilder Factory . newInstanceQ ; 
DocumentBuilder docBuilder = 

docBuilderFactory. newDocumentBuilderQ ; 
Document doc = docBuilder. parse(responseBody); 
method . releaseConnection () ; 



Making the Request: HTTP Libraries | 35 



// Hit the XML document with an XPath expression to get the list 
//of bookmarks. 

XPath xpath = XPathFactory.newInstance().newXPath(); 

NodeList bookmarks = (Nodetist)xpath.evaluate("/posts/post", doc, 

XPathConstants.NODESET); 

// Iterate over the bookmarks and print out each one. 
for (int i = 0; i < bookmarks. getl_ength(); i++) 
{ 

NamedNodeMap bookmark = bookmarks. item(i) .getAttributesQ; 
String description = bookmark. getNamedItem("description") 

.getl\lodeValue(); 
String uri = bookmark. getNamedItem("href ") .getNodeValueQ; 
System. out. println(description + ": " + uri); 

} 

System. exit(o); 

} 

} 

C#: System.Web.HTTPWebRequest 

The .NET Common Language Runtime (CLR) defines HTTPWebRequest for making 
HTTP requests, and NetworkCredential for authenticating the client to the server. The 
HTTPWebRequest constructor takes a URI. The NetworkCredential constructor takes a 
username and password (see Example 2-7). 

Example 2-7. A del.icio.us client in C# 

using System; 
using System. 10; 
using System. Net; 
using System. Xml. XPath; 

public class DeliciousApp { 

static string user = "username"; 
static string password = "password"; 

static Uri uri = new Uri("https://api.del.icio.us/vl/posts/recent"); 

static void Main(string[] args) { 

HttpWebRequest request = (HttpWebRequest) WebRequest.Create(uri); 
request. Credentials = new NetworkCredential(user, password); 
HttpWebResponse response = (HttpWebResponse) request. GetResponseQ; 

XPathDocument xml = new 

XPathDocument (response .Get ResponseStream( ) ) ; 

XPathNavigator navigator = xml.CreateNavigatorQ; 

foreach (XPathNavigator node in navigator. Select("/posts/post")) { 

string description = node.GetAttribute("description",""); 

string href = node.GetAttribute("href",""); 

Console. WriteLine(description + ": " + href); 

} 

} 

} 



36 | Chapter 2: Writing Web Service Clients 



PHP: libcurl 



PHP comes with a binding to the C library libcurl, which can do pretty much anything 
you might want to do with a URI (see Example 2-8) . 

Example 2-8. A del.icio.us client in PHP 

<?php 
$user = "username"; 
Spassword = "password"; 

Srequest = curl_init(); 
curl_setopt($request, CURLOPTJJRL, 

'https://api.del.icio.us/vl/posts/recent' ); 
curl_setopt($request, CURLOPT_USERPWD, "$user:$password"); 
curl_setopt($request, CURLOPT_RETURNTRANSFER, true); 

Sresponse = curl_exec($request); 

$xml = simplexml_load_string($response); 

curl_close($request); 

foreach ($xml->post as $post) { 
print "$post[description] : $post[href ]\n"; 

} 

?> 



JavaScript: XMLHttpRequest 

If you're writing a web service client in JavaScript, you probably intend it to run inside 
a web browser as part of an Ajax application. All modern web browsers implement a 
HTTP client library for JavaScript called XMLHttpRequest. 

Because Ajax clients are developed differently from standalone clients, I've devoted an 
entire chapter to them: Chapter 11. The first example in that chapter is a del.icio.us 
client, so you can skip there right now without losing the flow of the examples. 

The Command Line: curl 

This example is a bit different: it doesn't use a programming language at all. A program 
called curl (http://curl.haxx.se/) is a capable HTTP client that runs from the Unix or 
Windows command line. It supports most HTTP methods, custom headers, several 
authentication mechanisms, proxies, compression, and many other features. You can 
use curl to do quick one-off HTTP requests, or use it in conjunction with shell scripts. 
Here's curl in action, grabbing a user's del.icio.us bookmarks: 

$ curl https : //username : password@api . del . icio. us/vl/posts/recent 

<?xml version='l.O' standalone='yes' ?> 
<posts tag="" user="username"> 

</posts> 



Making the Request: HTTP Libraries | 37 



Other Languages 

I don't have the space or the expertise to cover every popular programming language 
in depth with a del.icio.us client example. I can, however, give brief pointers to HTTP 
client libraries for some of the many languages I haven't covered yet. 

ActionScript 

Flash applications, like JavaScript applications, generally run inside a web browser. 
This means that when you write an ActionScript web service client you'll probably 
use the Ajax architecture described in Chapter 11, rather than the standalone ar- 
chitecture shown in this chapter. 

ActionScript's XML class gives functionality similar to JavaScript's XmlHttpRequest. 
The XML. load method fetches a URI and parses the response document into an 
XML data structure. ActionScript also provides a class called LoadVars, which 
works on form-encoded key-value pairs instead of on XML documents. 

C 

The libwww library for C was the very first HTTP client library, but most C pro- 
grammers today use libcurl (http://curl.haxx.se/libcurl/) , the basis for the curl com- 
mand-line tool. Earlier I mentioned PHP's bindings to libcurl, but there are also 
bindings for more than 30 other languages. If you don't like my recommendations, 
or I don't mention your favorite programming language in this chapter, you might 
look at using the libcurl bindings. 

C++ 

Use libcurl, either directly or through an object-oriented wrapper called cURLpp 
(http://rrette.com/curlpp.html). 

Common Lisp 

simple-http (http://www.enterpriselisp.com/software/simple-httpf) is easy to use, 
but doesn't support anything but basic HTTP GET and POST. The AllegroServe 
web server library (http://opensource.franz.com/aservef) includes a complete HTTP 
client library. 

Perl 

The standard HTTP library for Perl is libwww-perl (also known as LWP), available 
from CPAN or most Unix packaging systems, libwww-perl has a long history and 
is one of the best-regarded Perl libraries. To get HTTPS support, you should also 
install the Crypt :SSLeay module (available from CPAN). 

Processing the Response: XML Parsers 

The entity-body is usually the most important part of an HTTP response. Where web 
services are concerned, the entity-body is usually an XML document, and the client 
gets most of the information it needs by running this document through an XML parser. 



38 | Chapter 2: Writing Web Service Clients 



Now, there are many HTTP client libraries, but they all have exactly the same task. 
Given a URI, a set of headers, and a body document, the client's job is to construct an 
HTTP request and send it to a certain server. Some libraries have more features than 
others: cookies, authentication, caching, and the other ones I mentioned. But all these 
extra features are implemented within the HTTP request, usually as extra headers. A 
library might offer an object-oriented interface (like Net: :HTTP) or a file-like interface 
(like open-uri), but both interfaces do the same thing. There's only one kind of HTTP 
client library. 

But there are three kinds of XML parsers. It's not just that some XML parsers have 
features that others lack, or that one interface is more natural than another. There are 
two basic XML parsing strategies: the document-based strategy of DOM and other tree- 
style parsers, and the event-based strategy of SAX and "pull" parsers. You can get a 
tree-style or a SAX parser for any programming language, and a pull parser for almost 
any language. 

The document-based, tree-style strategy is the simplest of the three models. A tree-style 
parser models an XML document as a nested data structure. Once you've got this data 
structure, you can search and process it with XPath queries, CSS selectors, or custom 
navigation functions: whatever your parser supports. A DOM parser is a tree-style 
parser that implements a specific interface defined by the W3C. 

The tree-style strategy is easy to use, and it's the one I use the most. With a tree-style 
parser, the document is just an object like the other objects in your program. The big 
shortcoming is that you have to deal with the document as a whole. You can't start 
working on the document until you've processed the whole thing into a tree, and you 
can't avoid loading the whole document into memory. For documents that are simple 
but very large, this is inefficient. It would be a lot better to handle tags as they're parsed. 

Instead of a data structure, a SAX-style or pull parser turns a document into a stream 
of events. Starting and closing tags, XML comments, and entity declarations are all 
events. 

A pull parser is useful when you need to handle almost every event. A pull parser lets 
you handle one event at a time, "pulling" the next one from the stream as needed. You 
can take action in response to individual events as they come in, or build up a data 
structure for later use — presumably a smaller data structure than the one a tree-style 
parser would build. You can stop parsing the document at any time and come back to 
it later by pulling the next event from the stream. 

A SAX parser is more complex, but useful when you only care about a few of the many 
events that will be streaming in. You drive a SAX parser by registering callback methods 
with it. Once you're done defining callbacks, you set the parser loose on a document. 
The parser turns the document into a series of events, and processes every event in the 
document without stopping. When an event comes along that matches one of your 
callbacks, the parser triggers that callback, and your custom code runs. Once the call- 
back completes, the SAX parser goes back to processing events without stopping. 



Processing the Response: XML Parsers | 39 



The advantage of the document-based approach is that it gives you random access to 
the document's contents. With event-based parsers, once the events have fired, they're 
gone. If you want to trigger them again you need to re-parse the document. What's 
more, an event-based parser won't notice that a malformed XML document is mal- 
formed until it tries to parse the bad spot, and crashes. Before passing a document into 
an event-based parser, you'll need to make sure the document is well formed, or else 
accept that your callback methods can be triggered for a document that turns out not 
to be good. 

Some programming languages come with a standard set of XML parsers. Others have 
a canonical third -party parser library. For the sake of performance, some languages also 
have bindings to fast parsers written in C. I'd like to go through the list of languages 
again now, and make recommendations for document- and event-based XML parsers. 
I'll rate commonly available parsers on speed, the quality of their interface, how well 
they support XPath (for tree-style parsers) , how strict they are, and whether or not they 
support schema-based validation. Depending on the application, a strict parser may be 
a good thing (because an XML document will be parsed the correct way or not at all) 
or a bad thing (because you want to use a service that generates bad XML). 

In the sample del.icio.us clients given above, I showed not only how to use my favorite 
HTTP client library for a language, but how to use my favorite tree-style parser for that 
language. To show you how event-based parsers work, I'll give two more examples of 
del.icio.us clients using Ruby's built-in SAX and pull parsers. 

Ruby: REXML, I Guess 

Ruby comes with a standard XML parser library, REXML, that supports both DOM 
and SAX interfaces, and has good XPath support. Unfortunately, REXML's internals 
put it in a strange middle ground: it's too strict to be used to parse bad XML, but not 
strict enough to reject all bad XML. 

I use REXML throughout this book because it's the default choice, and because I only 
deal with well-formed XML. If you want to guarantee that you only deal with well- 
formed XML, you'll need to install the Ruby bindings to the GNOME project's 
libxml2 library (described in "Other Languages" later in this chapter). 

If you want to be able to handle bad markup, the best choice is hpricot (http://code.why 
theluckystiff.net/hpricot/), available as the hpricot gem. It's fast (it uses a C extension), 
and it has an intuitive interface including support for common XPath expressions. 

Example 2-9 is an implementation of the del.icio.us client using REXML's SAX 
interface. 

Example 2-9. A Ruby client using a SAX parser 

#!/usr/bin/ruby -w 
# delicious-sax. rb 
require 'open-uri' 



40 | Chapter 2: Writing Web Service Clients 



require ' rexml/parsers/ sax2parser ' 

def print_my_recent_bookmarks(username, password) 

# Make an HTTPS request and read the entity-body as an XML document, 
xml = open( ' https://api.del.icio.us/vl/posts/recent ' , 

:http_basic_authentication => [username, password]) 

# Create a SAX parser whose destiny is to parse the XML entity-body, 
parser = REXML: : Parsers : :SAX2Parser.new(xml) 

# When the SAX parser encounters a 'post' tag... 

parser. listen (:start_element, ["post"]) do |uri, tag, fqtag, attributes! 

# ...it should print out information about the tag. 

puts "#{attributes[' description']}: #{attributes[ ' href ' ] }" 
end 

# Make the parser fulfil its destiny to parse the XML entity-body, 
parser. parse 

end 

# Main program, 
username, password = ARCV 
unless username and password 

puts "Usage: #{$0} [USERNAME] [PASSWORD]" 
exit 
end 

print_my_recent_bookmarks (username, password) 

In this program, the data isn't parsed (or even read from the HTTP connection) until 
the call to SAXParser#parse. Up to that point I'm free to call listen and set up pieces 
of code to run in response to parser events. In this case, the only event I'm interested 
in is the start of a post tag. My code block gets called every time the parser finds a 
post tag. This is the same as parsing the XML document with a tree-style parser, and 
running the XPath expression "//post" against the object tree. What does my code 
block do? The same thing my other example programs do when they find a post tag: 
print out the values of the description and href attributes. 

This implementation is faster and much more memory-efficient than the equivalent 
tree-style implementation. However, complex SAX-based programs are much more 
difficult to write than equivalent tree-style programs. Pull parsers are a good compro- 
mise. Example 2-10 shows a client implementation that uses REXML's pull parser 
interface. 

Example 2-10. A del.icio.us client using REXML's pull parser 

#!/usr/bin/ruby -w 

# delicious-pull. rb 
require 'open-uri' 

require ' rexml/parsers/ pullparser ' 

def print_my_recent_bookmarks(username, password) 

# Make an HTTPS request and read the entity-body as an XML document, 
xml = open('https://api.del.icio.us/vl/posts/recent', 



Processing the Response: XML Parsers | 41 



:http_basic_authentication => [username, password]) 

# Feed the XML entity-body into a pull parser 
parser = REXML: :Parsers: :PullParser.new(xml) 

# Until there are no more events to pull... 
while parser. has_next? 

# ...pull the next event, 
tag = parser. pull 

# If it ' s a ' post' tag. . . 
if tag.start_element? 

if tag[o] == 'post' 
# Print information about the bookmark, 
attrs = tag[l] 

puts "#{attrs[ 'description']}: #{attrs[ ' href ' ] }" 
end 
end 
end 
end 

# Main program, 
username, password = ARCV 
unless username and password 

puts "Usage: #{$0} [USERNAME] [PASSWORD]" 

exit 
end 

print_my_recent_bookmarks (username, password) 

Python: ElementTree 

The world is full of XML parsers for Python. There are seven different XML interfaces 
in the Python 2.5 standard library alone. For full details, see the Python library refer- 
ence (http://docs.python.org/lib/markup.html) . 

For tree-style parsing, the best library is ElementTree (http://effbot.org/zone/element- 
index.htm). It's fast, it has a sensible interface, and as of Python 2.5 you don't have to 
install anything because it's in the standard library. On the downside, its support for 
XPath is limited to simple expressions — of course, nothing else in the standard library 
supports XPath at all. If you need full XPath support, try 4Suite (http://4suite.org/) . 

Beautiful Soup (http://www.crummy.com/software/BeautifulSoup/) is a slower tree-style 
parser that is very forgiving of invalid XML, and offers a programmatic interface to a 
document. It also handles most character set conversions automatically, letting you 
work with Unicode data. 

For SAX-style parsing, the best choice is the xml.sax module in the standard library. 
The PyXML (http://pyxml.sourceforge.net/) suite includes a pull parser. 



42 | Chapter 2: Writing Web Service Clients 



Java: javax.xml, Xerces, or XMLPull 

Java 1.5 includes the XML parser written by the Apache Xerces project. The core classes 
are found in the packages javax.xml.*, (for instance, javax.xml. xpath). The DOM in- 
terface lives in org.w3c.dom.*, and the SAX interface lives in org.xml.sax.*. If you're 
using a previous version of Java, you can install Xerces yourself and take advantage of 
the same interface found in Java 1.5 (http://xerces.apache.org/xerces2-j/). 

There are a variety of pull parsers for Java. Sun's Web Services Developer Pack includes 
a pull parser in the javax.xml. stream package. 

For parsing bad XML, you might try TagSoup (http://home.ccil.org/~cowan/XML/ 
tagsoupf). 

C#: System.Xml.XmlReader 

The.NET Common Language Runtime comes with a pull parser interface, in contrast 
to the more typical (and more complex) SAX-style interface. You can also create a full 
W3C DOM tree using XmlDocument . The XPathDocument class lets you iterate over nodes 
in the tree that match an XPath expression. 

If you need to handle broken XML documents, check out Chris Lovett's SgmlReader at 
http://www.gotdotnet.com/Community/UserSamples/. 

PHP 

You can create a SAX-style parser with the function xml_parser_create, and a pull 
parser with the XMLReader extension. The DOM PHP extension (included in PHP 5) pro- 
vides a tree-style interface to the GNOME project's libxml2 C library. You might have 
an easier time using SimpleXML, a tree-style parser that's not an official DOM imple- 
mentation. That's what I used in Example 2-8. 

There's also a pure PHP DOM parser called DOMIT! (http://sourceforge.net/projects/ 
domit-xmlparser) . 

JavaScript: responseXML 

If you're using XMLHttpRequest to write an Ajax client, you don't have to worry about 
the XML parser at all. If you make a request and the response entity-body is in XML 
format, the web browser parses it with its own tree-style parser, and makes it available 
through the responseXML property of the XMLHttpRequest object. You manipulate this 
document with JavaScript DOM methods: the same ones you use to manipulate HTML 
documents displayed in the browser. Chapter 11 has more information on how to use 
responseXML — and how to handle non-XML documents with the responseData member. 



Processing the Response: XML Parsers | 43 



There's a third-party XML parser, XML for <SCRIPT> (http://xmljs.sourceforge.net/), 
which works independently of the parser built into the client's web browser. "XML for 
<SCRIPT>" offers DOM and SAX interfaces, and supports XPath queries. 

Other Languages 

ActionS cript 

When you load a URI with XML. load, it's automatically parsed into an XML object, 
which exposes a tree-style interface. 

C 

Expat (http://expat.sourceforge.net/) is the most popular SAX-style parser. The 
GNOME project's libxml2 (http://xmlsoft.org/) contains DOM, pull, and SAX 
parsers. 

C++ 

You can use either of the C parsers, or the object-oriented Xerces-C++ parser 
(http://xml.apache.org/xerces-c/). Like the Java version of Xerces, Xerces-C++ ex- 
poses both DOM and SAX interfaces. 

Common Lisp 

Use SXML (http://common-lisp.net/project/s-xml/). It exposes a SAX-like interface, 
and can also turn an XML document into tree-like S-expressions or Lisp data 
structures. 

Perl 

As with Python, there are a variety of XML parsers for Perl. They're all available 
on CP AN. XML: :XPath has XPath support, and XML: : Simple turns an XML docu- 
ment into standard Perl data structures. For SAX-style parsing, use 
XML: :SAX: :PurePerl. For pull parsing, use XML: :LibXML: : Reader. The Perl XML 
FAQ (http://perl-xml.sourceforge.net/faq/) has an overview of the most popular Perl 
XML libraries. 

JSON Parsers: Handling Serialized Data 

Most web services return XML documents, but a growing number return simple data 
structures (numbers, arrays, hashes, and so on), serialized as JSON-formatted strings. 
JSON is usually produced by services that expect to be consumed by the client half of 
an Ajax application. The idea is that it's a lot easier for a browser to get a JavaScript 
data structure from a JSON data structure than from an XML document. Every web 
browser offers a slightly different JavaScript interface to its XML parser, but a JSON 
string is nothing but a tightly constrained JavaScript program, so it works the same way 
in every browser. 

Of course, JSON is not tied to JavaScript, any more than JavaScript is to Java. JSON 
makes a lightweight alternative to XML-based approaches to data serialization, like 
XML Schema. The JSON web site (http://www.json.org/) links to implementations in 



44 | Chapter 2: Writing Web Service Clients 



many languages, and I refer you to that site rather than mentioning a JSON library for 
every language. 

JSON is a simple and language-independent way of formatting programming language 
data structures (numbers, arrays, hashes, and so on) as strings. Example 2-11 is a JSON 
representation of a simple data structure: a mixed-type array. 

Example 2-11. A mixed-type array in JSON format 
[3, "three"] 

By comparison, Example 2-12 is one possible XML representation of the same data. 

Example 2-12. A mixed-type array in XML-RPC format 

<value> 
<array> 
<data> 

<valuexi4>3</i4></value> 
<valuexstring>three</stringx/value> 
</data> 
</array> 
</value> 

Since a JSON string is nothing but a tightly constrained JavaScript program, you can 
"parse" JSON simply by calling eval on the string. This is very fast, but you shouldn't 
do it unless you control the web service that served your JSON. An untested or un- 
trusted web service can send the client buggy or malicious JavaScript programs instead 
of real JSON structures. For the JavaScript examples in Chapter 11,1 use a JSON parser 
written in JavaScript and available fromjson.org (see Example 2-13). 

Example 2-13. A JSON demo in JavaScript 
<!-- json-demo.html --> 

<!-- In a real application, you would save json.js locally 

instead of fetching it from json.org every time. --> 
<script type=" text/ javascript" src="http : //www. json.org/json. js"> 
</script> 

<script type="text/javascript"> 
array = [3, "three"] 

alert ("Converted array into DSON string: "' + array. toJSONStringQ + ""') 
json = "[4, \"four\"]" 

alert ("Converted DSON "' + json + "' into array:") 

array2 = json. parse} SON () 

for (i=0; i < array2. length; i++) 

{ 

alert("Element #" + i + " is " + array2[i]) 

} 

</script> 

The Dojo JavaScript framework has a JSON library in the dojo. json package, so if 
you're using Dojo you don't have to install anything extra. A future version of the 



JSON Parsers: Handling Serialized Data | 45 



ECMAScript standard may define JSON serialization and deserialization methods as 
part of the JavaScript language, making third-party libraries obsolete. 

In this book's Ruby examples, I'll use the JSON parser that comes from the json Ruby 
gem. The two most important methods are Object#to_json and DSON. parse. Try run- 
ning the Ruby code in Example 2-14 through the irb interpreter. 

Example 2-14. A JSON demo in Ruby 

# json-demo.rb 
require 'rubygems' 
require 'json' 

[3, "three"]. to Json # => "[3,\"three\"]" 

JSON.parse(' [4, "four"]') # => [4, "four"] 

Right now, Yahoo! Web Services are the most popular public web services to serve 
JSON (http://developer.yahoo.com/common/json.html). Example 2-15 shows a com- 
mand-line program, written in Ruby, that uses the Yahoo! News web service to get a 
JSON representation of current news stories. 

Example 2-15. Searching the Web with Yahool's web service (JSON edition) 

#!/usr/bin/ruby 

# yahoo-web-search-json.rb 
require 'rubygems' 
require 'json' 

require 'open-uri' 
$KC0DE = 'UTF8' 

# Search the web for a term, and print the titles of matching web pages, 
def search (term) 

base_uri = ' http : //api . search . yahoo . com/NewsSearchService/Vl/newsSearch ' 

# Make the HTTP request and read the response entity-body as a DSON 

# document. 

json = open(base_uri + "?appid=restbook&output=json&query=#{term}") .read 

# Parse the DSON document into a Ruby data structure, 
json = DSON.parse(json) 

# Iterate over the data structure... 
json[ ' ResultSet ' ] [ ' Result' ] .each do 

# ...and print the title of each web page. 
| xr | puts r[ 'Title'] 
end 
end 

# Main program, 
unless ARGV[0] 

puts "Usage: #{$0} [search term]" 
exit 
end 

search(ARGV[0]) 



46 | Chapter 2: Writing Web Service Clients 



Compare this to the program yahoo-web-search.rb in Example 2-1. That program has 
the same basic structure, but it works differently. It asks for search results formatted 
as XML, parses the XML, and uses an XPath query to extract the result titles. This 
program parses a JSON data structure into a native-language data structure (a hash), 
and traverses it with native-language operators instead of XPath. 

If JSON is so simple, why not use it for everything? You could do that, but I don't 
recommend it. JSON is good for representing data structures in general, and the Web 
mainly serves documents: irregular, self-describing data structures that link to each 
other. XML and HTML are specialized for representing documents. A JSON represen- 
tation of a web page would be hard to read, j ust like the XML representation of an array 
in Example 2-12 was hard to read. JSON is useful when you need to describe a data 
structure that doesn't fit easily into the document paradigm: a simple list, for instance, 
or a hash. 

Clients Made Easy with WADL 

So far I've presented code in a variety of languages, but it always follows the same three- 
step pattern. To call a web service I build up the elements of an HTTP request (method, 
URI, headers, and entity-body) . I use an HTTP library to turn that data into a real HTTP 
request, and the library sends the request to the appropriate server. Then I use an XML 
parser to parse the response into a data structure or a series of events. Once I make the 
request, I'm free to use the response data however I like. In this regard all RESTful web 
services, and most hybrid services, are the same. What's more, as I'll show in the chap- 
ters to come, all RESTful web services use HTTP the same way: HTTP has what's called 
a uniform interface. 

Can I take advantage of this similarity? Abstract this pattern out into a generic "REST 
library" that can access any web service that supports the uniform interface? There's 
precedent for this. The Web Service Description Language (WSDL) describes the dif- 
ferences between RPC-style web services in enough detail that a generic library can 
access any RPC-style SOAP service, given an appropriate WSDL file. 

For RESTful and hybrid services, I recommend using the Web Application Description 
Language. A WADL file describes the HTTP requests you can legitimately make of a 
service: which URIs you can visit, what data those URIs expect you to send, and what 
data they serve in return. A WADL library can parse this file and model the space of 
possible service requests as a native language API. 

I describe WADL in more detail in Chapter 9, but here's a taste. The del.icio.us client 
shown in Example 2-16 is equivalent to the Ruby client in Example 2-4, but it uses 
Ruby's WADL library and a bootleg WADL file I created for del.icio.us. (I'll show you 
the WADL file in Chapter 8.) 



Clients Made Easy with WADL | 47 



Example 2-16. A Ruby/WADL client for delicious 

#!/usr/bin/ruby 

# delicious-wadl-ruby.rb 
require 'wadl' 

if ARGV.size != 2 

puts "Usage: #{$0} [username] [password]" 

exit 
end 

username, password = ARGV 

# Load an application from the WADL file 

delicious = WADL: Application. from_wadl(open("delicious. wadl")) 

# Give authentication information to the application 
service = delicious. vl.with_basic_auth(username, password) 

begin 

# Find the "recent posts" functionality 
recent_posts = service. posts. recent 

# For every recent post... 

recent_posts. get. representation. each_by_param( ' post') do | post | 
# Print its description and URI. 

puts "#{post.attributes[ 'description' ]}: #{post.attributes[ 'href ]}" 
end 

rescue WADL: : Faults: :AuthorizationRequired 

puts "Invalid authentication information!" 
end 

Behind the scenes, this code makes exactly the same HTTP request as the other 
del.icio.us clients seen in this chapter. The details are hidden in the WADL file deli 
cious.wadl, which is interpreted by the WADL client library inside 
WADL: :Application.from_WADL. This code is not immediately recognizable as a web 
service client. That's a good thing: it means the library is doing its job. And yet, when 
we come back to this code in Chapter 9, you'll see that it follows the principles of REST 
as much as the examples that made their own HTTP requests. WADL abstracts away 
the details of HTTP, but not the underlying RESTful interface. 

As of the time of writing, WADL adoption is very poor. If you want to use a WADL 
client for a service, instead of writing a language-specific client, you'll probably have 
to write the WADL file yourself. It's not difficult to write a bootleg WADL file for 
someone else's service: I've done it for del.icio.us and a few other services. You can 
even write a WADL file that lets you use a web application — designed for human use 
— as a web service. WADL is designed to describe RESTful web services, but it can 
describe almost anything that goes on the Web. 

A Ruby library called ActiveResource takes a different strategy. It only works with 
certain kinds of web services, but it hides the details of RESTful HTTP access behind 
a simple object-oriented interface. I cover ActiveResource in the next chapter, after 
introducing some REST terminology. 



48 | Chapter 2: Writing Web Service Clients 



CHAPTER 3 

What Makes RESTful Services 

Different? 



I pulled a kind of bait-and-switch on you earlier, and it's time to make things right. 
Though this is a book about RESTful web services, most of the real services I've shown 
you are REST-RPC hybrids like the del.icio.us API: services that don't quite work like 
the rest of the Web. This is because right now, there just aren't many well-known 
RESTful services that work like the Web. In previous chapters I wanted to show you 
clients for real services you might have heard of, so I had to take what I could get. 

The del.icio.us and Flickr APIs are good examples of hybrid services. They work like 
the Web when you're fetching data, but they're RPC-style services when it comes time 
to modify the data. The various Yahoo! search services are very RESTful, but they're 
so simple that they don't make good examples. The Amazon E-Commerce Service (seen 
in Example 1-2) is also quite simple, and defects to the RPC style on a few obscure but 
important points. 

These services are all useful. I think the RPC style is the wrong one for web services, 
but that never prevents me from writing an RPC-style client if there's interesting data 
on the other side. I can't use Flickr or the del.icio.us API as examples of how to de- 
sign RESTful web services, though. That's why I covered them early in the book, when 
the only thing I was trying to show was what's on the programmable web and how to 
write HTTP clients. Now that we're approaching a heavy design chapter, I need to show 
you what a service looks like when it's RESTful and resource-oriented. 

Introducing the Simple Storage Service 

Two popular web services can answer this call: the Atom Publishing Protocol (APP), 
and Amazon's Simple Storage Service (S3). (Appendix A lists some publicly deployed 
RESTful web services, many of which you may not have heard of.) The APP is less an 
actual service than a set of instructions for building a service, so I'm going to start with 
S3, which actually exists at a specific place on the Web. In Chapter 9 I discuss the APP, 



49 



Atom, and related topics like Google's GData. For much of the rest of this chapter, I'll 
explore S3. 

S3 is a way of storing any data you like, structured however you like. You can keep 
your data private, or make it accessible by anyone with a web browser or BitTorrent 
client. Amazon hosts the storage and the bandwidth, and charges you by the gigabyte 
for both. To use the example S3 code in this chapter, you'll need to sign up for the S3 
service by going to http://aws.amazon.com/s3 . The S3 technical documentation is at 
http://docs.amazonwebservices.com/AmazonS3/2006-03-01/. 

There are two main uses for S3, as a: 

Backup server 

You store your data through S3 and don't give anyone else access to it. Rather than 
buying your own backup disks, you're renting disk space from Amazon. 

Data host 

You store your data on S3 and give others access to it. Amazon serves your data 
through HTTP or BitTorrent. Rather than paying an ISP for bandwidth, you're 
paying Amazon. Depending on your existing bandwidth costs this can save you a 
lot of money. Many of today's web startups use S3 to serve data files. 

Unlike the services I've shown so far, S3 is not inspired by any existing web site. The 
del.icio.us API is based on the del.icio.us web site, and the Yahoo! search services are 
based on corresponding web sites, but there's no web page on amazon.com where you 
fill out HTML forms to upload your files to S3. S3 is intended only for programmatic 
use. (Of course, if you use S3 as a data host, people will use it through their web 
browsers, without even knowing they're making a web service call. It'll act like a normal 
web site.) 

Amazon provides sample libraries for Ruby, Python, Java, C#, and Perl (see http:// 
developer. amazonwebservices.com/connectlkbcategory .]spa^categorylD=47) . There are 
also third-party libraries, like Ruby's AWS::S3 (http://amazon.rubyforge.org/) , which 
includes the s3sh shell I demonstrated back in Example 1-4. 

Object-Oriented Design of S3 

S3 is based on two concepts: S3 "buckets" and S3 "objects." An object is a named piece 
of data with some accompanying metadata. A bucket is a named container for objects. 
A bucket is analogous to the filesystem on your hard drive, and an object to one of the 
files on that filesystem. It's tempting to compare a bucket to a directory on a filesystem, 
but filesystem directories can be nested and buckets can't. If you want a directory 
structure inside your bucket, you need to simulate one by giving your objects names 
like "directory/subdirectory/file-object." 



50 | Chapter 3: What Makes RESTful Services Different? 



A Few Words About Buckets 



A bucket has one piece of information associated with it: the name. A bucket name can 
only contain the characters A through Z, a through z, through 9, underscore, period, 
and dash. I recommend staying away from uppercase letters in bucket names. 

As I mentioned above, buckets cannot contain other buckets: only objects. Each S3 
user is limited to 100 buckets, and your bucket name cannot conflict with anyone else's. 
I recommend you either keep everything in one bucket, or name each bucket after one 
of your projects or domain names. 

A Few Words About Objects 

An object has four parts to it: 

• A reference to the parent bucket. 

• The data stored in that object (S3 calls this the "value"). 

• A name (S3 calls it the "key"). 

• A set of metadata key-value pairs associated with the object. This is mostly custom 
metadata, but it may also include values for the standard HTTP headers Content - 
Type and Content-Disposition. 

If I wanted to host the O'Reilly web site on S3, I'd create a bucket called "oreilly.com," 
and fill it with objects whose keys were "" (the empty string), "catalog," "catalog/ 
9780596529260," and so on. These objects correspond to the URIs http://oreil 
ly.com/, http://oreilly.com/catalog, and so on. The object's values would be the HTML 
contents of O'Reilly's web pages. These S3 objects would have their Content-Type met- 
adata value set to text/html, so that people browsing the site would be served these 
objects as HTML documents, as opposed to XML or plain text. 

What If S3 Was a Standalone Library? 

If S3 was implemented as an object-oriented code library instead of a web service, you'd 
have two classes S3Bucket and S30bject. They'd have getter and setter methods for their 
data members: S3Bucket#name, S30bject.value=, S3Bucket#add0bject, and the like. The 
S3Bucket class would have an instance method S3Bucket#get0bjects that returned a list 
of S30bject instances, and a class method S3Bucket.getBuckets that returned all of your 
buckets. Example 3-1 shows what the Ruby code for this class might look like. 

Example 3-1. S3 implemented as a hypothetical Ruby library 

class S3Bucket 

# A class method to fetch all of your buckets, 
def self .getBuckets 

end 

# An instance method to fetch the objects in a bucket. 



Object-Oriented Design of S3 | 51 



def getObjects 
end 

end 

class S30bject 

# Fetch the data associated with this object, 
def data 

end 

# Set the data associated with this object, 
def data=(new_value) 

end 

end 

Resources 

Amazon exposes S3 as two different web services: a RESTful service based on plain 
HTTP envelopes, and an RPC-style service based on SOAP envelopes. The RPC-style 
service exposes functions much like the methods in Example 3-1 's hypothetical Ruby 
library: ListAHHyBuckets, CreateBucket, and so on. Indeed, many RPC-style web serv- 
ices are automatically generated from their implementation methods, and expose the 
same interfaces as the programming-language code they call behind the scenes. This 
works because most modern programming (including object-oriented programming) 
is procedural. 

The RESTful S3 service exposes all the functionality of the RPC-style service, but in- 
stead of doing it with custom-named functions, it exposes standard HTTP objects 
called resources. Instead of responding to custom method names like getObjects, a 
resource responds to one or more of the six standard HTTP methods: GET, HEAD, 
POST, PUT, DELETE, and OPTIONS. 

The RESTful S3 service provides three types of resources. Here they are, with sample 
URIs for each: 

• The list of your buckets (https://s3.amazonaws.com/). There's only one resource 
of this type. 

• A particular bucket (https://s3.amazonaws.eom/fnoffle-o/-fcuc/cet//). There can be 
up to 100 resources of this type. 

• A particular S3 object inside a bucket (https://s3.amazonaws.com/fname-o/- 
bucket) / {name-of -object}) . There can be infinitely many resources of this type. 

Each method from my hypothetical object-oriented S3 library corresponds to one of 
the six standard methods on one of these three types of resources. The getter method 
S30bject#name corresponds to a GET request on an "S3 object" resource, and the setter 
method S30bject#value= corresponds to a PUT request on the same resource. Factory 



52 I Chapter 3: What Makes RESTful Services Different? 



methods like S3Bucket.getBuckets and relational methods like S3Bucket#get0bjects 

correspond to GET methods on the "bucket list" and "bucket" resources. 

Every resource exposes the same interface and works the same way. To get an object's 
value you send a GET request to that object's URI. To get only the metadata for an 
object you send a HEAD request to the same URI. To create a bucket, you send a PUT 
request to a URI that incorporates the name of the bucket. To add an object to a bucket, 
you send PUT to a URI that incorporates the bucket name and object name. To delete 
a bucket or an object, you send a DELETE request to its URI. 

The S3 designers didn't just make this up. According to the HTTP standard this is what 
GET, HEAD, PUT, and DELETE are for. These four methods (plus POST and OP- 
TIONS, which S3 doesn't use) suffice to describe all interaction with resources on the 
Web. To expose your programs as web services, you don't need to invent new vocab- 
ularies or smuggle method names into URIs, or do anything except think carefully about 
your resource design. Every REST web service, no matter how complex, supports the 
same basic operations. All the complexity lives in the resources. 

Table 3-1 shows what happens when you send an HTTP request to the URI of an S3 
resource. 

Table 3-1. S3 resources and their methods 

HEAD PUT DELETE 

Create the bucket Delete the bucket 

Get the object's Set the object's Delete the object 
metadata value and meta- 

data 



That table looks kind of ridiculous. Why did I take up valuable space by printing it? 
Everything just does what it says. And that is why I printed it. In a well-designed REST- 
ful service, everything does what it says. 

You may well be skeptical of this claim, given the evidence so far. S3 is a pretty generic 
service. If all you're doing is sticking data into named slots, then of course you can 
implement the service using only generic verbs like GET and PUT. In Chapter 5 and 
Chapter 6 I'll show you strategies for mapping any kind of action to the uniform in- 
terface. For a sample preconvincing, note that I was able to get rid of 
S3Bucket.getBuckets by defining a new resource as "the list of buckets," which re- 
sponds only to GET. Also note that S3Bucket#add0bject simply disappeared as a natural 
consequence of the resource design, which requires that every object be associated with 
some bucket. 

Compare this to S3's RPC-style SOAP interface. To get the bucket list through SOAP, 
the method name is ListAHMyBuckets. To get the contents of a bucket, the method 



GET 

The bucket list (/) List your buckets 

A bucket (/{bucket }) List the bucket's 

objects 

An object (/ '{bucket} / '{object}) Get the object's 

value and meta- 
data 



Resources | 53 



name is ListBucket. With the RESTful interface, it's always GET. In a RESTful service, 
the URI designates an object (in the object-oriented sense) and the method names are 
standardized. The same few methods work the same way across resources and services. 

HTTP Response Codes 

Another defining feature of a RESTful architecture is its use of HTTP response codes. 
If you send a request to S3, and S3 handles it with no problem, you'll probably get back 
an HTTP response code of 200 ("OK"), just like when you successfully fetch a web 
page in your browser. If something goes wrong, the response code will be in the 3xx, 
4xx, or 5xx range: for instance, 500 ("Internal Server Error"). An error response code 
is a signal to the client that the metadata and entity-body should not be interpreted as 
a response to the request. It's not what the client asked for: it's the server's attempt to 
tell the client about a problem. Since the response code isn't part of the document or 
the metadata, the client can see whether or not an error occurred just by looking at the 
first three bytes of the response. 

Example 3-2 shows a sample error response. I made an HTTP request for an object that 
didn't exist (https://s3.amazonaws.com/crummy.com/nonexistent/object). The re- 
sponse code is 404 ("Not Found"). 

Example 3-2. A sample error response from S3 
404 Not Found 

Content-Type: application/xml 
Date: Fri, 10 Nov 2006 20:04:45 GMT 
Server: AmazonS3 
Transfer- Encoding: chunked 

X-amz-id-2: /sBIPQxmCsyRXDwGWNzxut5P+K96/Wvx4FhvVACbjRfNbhbDyBH5RC511sIzOwO 
X-amz-request-id: ED2168503ABB7BF4 

<?xml version="l.0" encoding="UTF-8"?> 
<Error> 
<Code>NoSuchKey</Code> 

<Message>The specified key does not exist. </Message> 

<Key>nonexistent/object</Key> 

<RequestId>ED2l68503ABB7BF4</RequestId> 

<HostId>/sBIPQxHDCsyRX]wGWNzxuL5P+K96/Wvx4FhvVACbjRfNbhbDyBH5RC511sIzOwO</HostId> 
</Error> 

HTTP response codes are underused on the human web. Your browser doesn't show 
you the HTTP response code when you request a page, because who wants to look at 
a numeric code when you can just look at the document to see whether something went 
wrong? When an error occurs in a web application, most web applications send 200 
("OK") along with a human-readable document that talks about the error. There's very 
little chance a human will mistake the error document for the document they requested. 

On the programmable web, it's just the opposite. Computer programs are good at 
taking different paths based on the value of a numeric variable, and very bad at figuring 



54 I Chapter 3: What Makes RESTful Services Different? 



out what a document "means." In the absence of prearranged rules, there's no way for 
a program to tell whether an XML document contains data or describes an error. HTTP 
response codes are the rules: rough conventions about how the client should approach 
an HTTP response. Because they're not part of the entity-body or metadata, a client 
can understand what happened even if it has no clue how to read the response. 

S3 uses a variety of response codes in addition to 200 ("OK") and 404 ("Not Found") . 
The most common is probably 403 ("Forbidden"), used when the client makes a re- 
quest without providing the right credentials. S3 also uses a few others, including 400 
("Bad Request") , which indicates that the server couldn't understand the data the client 
sent; and 409 ("Conflict"), sent when the client tries to delete a bucket that's not empty. 
For a full list, see the S3 technical documentation under "The REST Error Response." 
I describe every HTTP response code in Appendix B, with a focus on their application 
to web services. There are 41 official HTTP response codes, but only about 10 are 
important in everyday use. 

An S3 Client 

The Amazon sample libraries, and the third-party contributions like AWS::S3, elimi- 
nate much of the need for custom S3 client libraries. But Fm not telling you about S3 
just so you'll know about a useful web service. I want to use it to illustrate the theory 
behind REST. So Fm going to write a Ruby S3 client of my own, and dissect it for you 
as I go along. 

Just to show it can be done, my library will implement an object-oriented interface, like 
the one from Example 3-1, on top of the S3 service. The result will look like ActiveRe- 
cord or some other object-relational mapper. Instead of making SQL calls under the 
covers to store data in a database, though, it'll make HTTP requests under the covers 
to store data on the S3 service. Rather than give my methods resource-specific names 
like getBuckets and getObjects, I'll try to use names that reflect the underlying RESTful 
interface: get, put, and so on. 

The first thing I need is an interface to Amazon's rather unusual web service authori- 
zation mechanism. But that's not as interesting as seeing the web service in action, so 
I'm going to skip it for now. I'm going to create a very small Ruby module called 
S3: : Authorized, just so my other S3 classes can include it. I'll come back to it at the 
end, and fill in the details. 

Example 3-3 shows a bit of throat-clearing code. 

Example 3-3. S3 Ruby client: Initial code 

#!/usr/bin/ruby -w 

# S3lib.rb 

# Libraries necessary for making HTTP requests and parsing responses, 
require 'rubygems' 

require 'rest-open-uri' 



An S3 Client | 55 



require 'rexml/document' 



# Libraries necessary for request signing 
require 'openssl' 

require 'digest/shal' 
require 'base64' 
require 'uri' 

module S3 # This is the beginning of a big, all-encompassing module. 

module Authorized 

# Enter your public key (Amazon calls it an "Access Key ID") and 

# your private key (Amazon calls it a "Secret Access Key"). This is 

# so you can sign your S3 requests and Amazon will know who to 

# charge. 
I«)@public_key = ' ' 
(5)@private_key = ' ' 

if @@public_key. empty? or |S@private_key. empty? 

raise "You need to set your S3 keys." 
end 

# You shouldn't need to change this unless you're using an S3 clone like 

# Park Place. 

HOST = 'https://s3.amazonaws.com/' 
end 

The only interesting aspect of this bare-bones S3: Authorized is that it's where you 
should plug in the two cryptographic keys associated with your Amazon Web Services 
account. Every S3 request you make includes your public key (Amazon calls it an 'Ac- 
cess Key ID") so that Amazon can identify you. Every request you make must be cryp- 
tographically signed with your private key (Amazon calls it a "Secret Access Key") so 
that Amazon knows it's really you. I'm using the standard cryptographic terms, even 
though your "private key" is not totally private — Amazon knows it too. It is private in 
the sense that you should never reveal it to anyone else. If you do, the person you reveal 
it to will be able to make S3 requests and have Amazon charge you for it. 

The Bucket List 

Example 3-4 shows an object-oriented class for my first resource, the list of buckets. 
I'll call the class for this resource S3 : : BucketList. 

Example 3-4. S3 Ruby client: the S3::BucketList class 

# The bucket list, 
class BucketList 

include Authorized 

# Fetch all the buckets this user has defined, 
def get 

buckets = [] 



56 | Chapter 3: What Makes RESTful Services Different? 



# GET the bucket list URI and read an XML document from it. 
doc = REXML: : Document. new(open (HOST) .read) 

# For every bucket. . . 

REXML: :XPath.each(doc, "//Bucket/Name") do |e| 
# ...create a new Bucket object and add it to the list, 
buckets << Bucket. new(e. text) if e.text 

end 

return buckets 
end 
end 



XPath Exposition 

Reading from right to left, the XPath expression //Bucket/Name means: 

Find every Name tag Name 
that's the direct child of a Bucket tag Bucket/ 
anywhere in the document. // 



Now my file is a real web service client. If I call S3: :BucketList#get I make a secure 
HTTP GET request to https://s3.amazonaws.com/, which happens to be the URI of the 
resource "a list of your buckets." The S3 service sends back an XML document that 
looks something like Example 3-5. This is a representation (as I'll start calling it in the 
next chapter) of the resource "a list of your buckets." It's just some information about 
the current state of that list. The Owner tag makes it clear whose bucket list it is (my 
AWS account name is evidently "leonardr28"), and the Buckets tag contains a number 
of Bucket tags describing my buckets (in this case, there's one Bucket tag and one 
bucket) . 

Example 3-5. A sample "list of your buckets" 
<?xml version='l.O' encoding='UTF-8' ?> 

<ListAllMyBucketsResult xmlns= ' http : //s3 . amazonaws . com/doc/2006-03-01/ ' > 
<0wner> 

<ID>c0363f7260f2f5fcf38d48039f4fb5cab21b0605778l7310be5170e7774aad70</ID> 

<DisplayName>leonardr28</DisplayName> 
</0wner> 
<Buckets> 

<Bucket> 
<Name>crummy.com</Name> 

<CreationDate>2006-10-26Tl8 :46 :45 .OOOZ</CreationDate> 
</Bucket> 
</Buckets> 
</ListAllMyBucketsResult> 

For purposes of this small client application, the Name is the only aspect of a bucket I'm 
interested in. The XPath expression //Bucket/Name gives me the name of every bucket, 
which is all I need to create Bucket objects. 



An S3 Client | 57 



As we'll see, one thing that's missing from this XML document is links. The document 
gives the name of every bucket, but says nothing about where the buckets can be found 
on the Web. In terms of the REST design criteria, this is the major shortcoming of 
Amazon S3. Fortunately, it's not too difficult to program a client to calculate a URI 
from the bucket name. I just follow the rule I gave earlier: https://s3.amazonaws.com/ 
{name-of -bucket}. 

The Bucket 

Now, as shown in Example 3-6, let's write the S3 : : Bucket class, so that 
S3: :BucketList.get will have something to instantiate. 

Example 3-6. S3 Ruby client: the S3::Bucket class 

# A bucket that you've stored (or will store) on the S3 application, 
class Bucket 

include Authorized 

attr_accessor :name 

def initialize(name) 

@name = name 
end 

# The URI to a bucket is the service root plus the bucket name, 
def uri 

HOST + URI.escape(name) 
end 

# Stores this bucket on S3. Analagous to ActiveRecord: :Base#save, 

# which stores an object in the database. See below in the 

# book text for a discussion of acl_policy. 
def put(acl_policy=nil) 

# Set the HTTP method as an argument to open(). Also set the S3 

# access policy for this bucket, if one was provided, 
args = {:method => :put} 

args["x-amz-acl"] = acl_policy if acl_policy 

# Send a PUT request to this bucket's URI. 
open(uri, args) 

return self 
end 

# Deletes this bucket. This will fail with HTTP status code 409 

# ("Conflict") unless the bucket is empty, 
def delete 

# Send a DELETE request to this bucket's URI. 
open(uri, :method => :delete) 

end 

Here are two more web service methods: S3 : : Bucket#put and S3 : : Bucket#delete. Since 
the URI to a bucket uniquely identifies the bucket, deletion is simple: you send a DE- 
LETE request to the bucket URI, and it's gone. Since a bucket's name goes into its URI, 



58 I Chapter 3: What Makes RESTful Services Different? 



and a bucket has no other settable properties, it's also easy to create a bucket: just send 
a PUT request to its URI. As I'll show when I write S3 : :0bject, a PUT request is more 
complicated when not all the data can be stored in the URI. 

Earlier I compared my S3 : : classes to ActiveRecord classes, but S3 : : Bucket#put works 
a little differently from an ActiveRecord implementation of save. A row in an Active- 
Record-controlled database table has a numeric unique ID. If you take an ActiveRecord 
object with ID 23 and change its name, your change is reflected as a change to the 
database record with ID 23: 

SET name="newname" WHERE id=23 

The permanent ID of an S3 bucket is its URI, and the URI includes the name. If you 
change the name of a bucket and call put, the client doesn't rename the old bucket on 
S3: it creates a new, empty bucket at a new URI with the new name. This is a result of 
design decisions made by the S3 programmers. It doesn't have to be this way. The Ruby 
on Rails framework has a different design: when it exposes database rows through a 
RESTful web service, the URI to a row incorporates its numeric database IDs. If S3 was 
a Rails service you'd see buckets at URIs like /buckets/23. Renaming the bucket 
wouldn't change the URI. 

Now comes the last method of S3:: Bucket, which I've called get. Like 
S3: :BucketList.get, this method makes a GET request to the URI of a resource (in this 
case, a "bucket" resource), fetches an XML document, and parses it into new instances 
of a Ruby class (see Example 3-7) . This method supports a variety of ways to filter the 
contents of S3 buckets. For instance, you can use : Prefix to retrieve only objects whose 
keys start with a certain string. I won't cover these filtering options in detail. If you're 
interested in them, see the S3 technical documentation on "Listing Keys." 

Example 3-7. S3 Ruby client: the S3::Bucket class (concluded) 

# Get the objects in this bucket: all of them, or some subset. 
# 

# If S3 decides not to return the whole bucket/subset, the second 

# return value will be set to true. To get the rest of the objects, 

# you'll need to manipulate the subset options (not covered in the 

# book text). 
# 

# The subset options are :Prefix, :Marker, delimiter, :MaxKeys. 

# For details, see the S3 docs on "Listing Keys", 
def get(options={}) 

# Get the base URI to this bucket, and append any subset options 

# onto the query string, 
uri = uri() 

suffix = ' ? ' 

# For every option the user provided... 
options. each do |param, value| 

# ...if it's one of the S3 subset options... 
if [: Prefix, : Marker, : Delimiter, :MaxKeys] .member? :param 
# ...add it to the URI. 

uri << suffix << param.to_s << '=' << URI.escape(value) 



An S3 Client | 59 



suffix = '&' 
end 
end 

# Now we've built up our URI. Make a GET request to that URI and 

# read an XML document that lists objects in the bucket, 
doc = REXML: : Document. new(open(uri) .read) 

there_are_more = REXML: :XPath.first(doc, "//IsTruncated") .text == "true" 

# Build a list of S3::0bject objects, 
objects = [] 

# For every object in the bucket... 

REXML: :XPath.each(doc, "//Contents/Key") do |e| 
# ...build an S3::0bject object and append it to the list, 
objects << Object. new(self, e.text) if e.text 

end 

return objects, there_are_more 
end 
end 



XPath Exposition 

Reading from right to left, the XPath expression //IsTruncated means: 

Find every IsTruncated tag IsTruncated 
anywhere in the document. // 



Make a GET request of the application's root URI, and you get a representation of the 
resource "a list of your buckets. " Make a GET request to the URI of a "bucket" resource, 
and you get a representation of the bucket: an XML document like the one in Exam- 
ple 3-8, containing a Contents tag for every element of the bucket. 

Example 3-8. A sample bucket representation 
<?xml version='l.O' encoding='UTF-8' ?> 

< List Bucket Result xmlns=" http://s3.amazonaws.com/doc/2006-03-Ol/"> 
<Name>crummy . com</Name> 
<Prefixx/Prefix> 
<Markerx/Marker> 
<MaxKeys>1000</MaxKeys> 
<IsTruncated>false</IsTruncated> 
<Contents> 

<Key>mydocument</Key> 

<LastModified>2006-10-27Tl6:01:l9.000Z</LastModified> 
<ETag>"93bede57fd38l8f93eedceOdef329cc7"</ETag> 
<Size>22</Size> 
<0wner> 
<ID> 

C0363f7260f2f5fcf38d48039f4fb5cab21b0605778l7310be5170e7774aad70</ID> 
<DisplayName>leonardr28</DisplayName> 
</0wner> 



60 I Chapter 3: What Makes RESTful Services Different? 



<StorageClass>STANDARD</StorageClass> 
</Contents> 
</ListBucketResult> 

In this case, the portion of the document I find interesting is the list of a bucket's objects. 
An object is identified by its key, and I use the XPath expression "//Contents/Key" to 
fetch that information. I'm also interested in a certain Boolean variable ("//IsTrunca- 
ted"): whether this document contains keys for every object in the bucket, or whether 
S3 decided there were too many to send in one document and truncated the list. 

Again, the main thing missing from this representation is links. The document lists lots 
of information about the objects, but not their URIs. The client is expected to know 
how to turn an object name into that object's URI. Fortunately, it's not too hard to 
build an object's URI, using the rule I already gave: https://s3.amazonaws.com/fname- 
of -bucket) I {name-of -object}. 

The S3 Object 

Now we're ready to implement an interface to the core of the S3 service: the object. 
Remember that an S3 object is just a data string that's been given a name (a key) and 
a set of metadata key-value pairs (such as Content-Type="text/html"). When you send 
a GET request to the bucket list, or to a bucket, S3 serves an XML document that you 
have to parse. When you send a GET request to an object, S3 serves whatever data 
string you PUT there earlier — byte for byte. 

Example 3-9 shows the beginning of S3 : : Object, which should be nothing new by now. 

Example 3-9. S3 Ruby client: the S3::Object class 

# An S3 object, associated with a bucket, containing a value and metadata, 
class Object 
include Authorized 

# The client can see which Bucket this Object is in. 
attr_reader : bucket 

# The client can read and write the name of this Object. 
attr_accessor :name 

# The client can write this Object's metadata and value. 

# I'll define the corresponding "read" methods later. 
attr_writer :metadata, :value 

def initialize(bucket, name, value=nil, metadata=nil) 

(Sbucket, (Sname, Rvalue, ^metadata = bucket, name, value, metadata 
end 

# The URI to an Object is the URI to its Bucket, and then its name, 
def uri 

(Sbucket.uri + '/' + URI.escape(name) 
end 



An S3 Client | 61 



What comes next is my first implementation of an HTTP HEAD request. I use it to 
fetch an object's metadata key-value pairs and populate the metadata hash with it (the 
actual implementation of storejnetadata comes at the end of this class) . Since I'm using 
rest-open-uri, the code to make the HEAD request looks the same as the code to make 
any other HTTP request (see Example 3-10). 

Example 3-10. S3 Ruby client: the S3::Object#metadata method 

# Retrieves the metadata hash for this Object, possibly fetching 

# it from S3, 
def metadata 

# If there's no metadata yet... 
unless (^metadata 

# Make a HEAD request to this Object's URI, and read the metadata 

# from the HTTP headers in the response, 
begin 

store_metadata(open(uri, :method => :head).meta) 
rescue OpenURI: :HTTPError => e 
if e.io. status == ["404", "Not Found"] 

# If the Object doesn't exist, there's no metadata and this is not 

# an error, 
^metadata = {} 

else 

# Otherwise, this is an error, 
raise e 

end 
end 

end 

return ^metadata 
end 

The goal here is to fetch an object's metadata without fetching the object itself. This is 
the difference between downloading a movie review and downloading the movie, and 
when you're paying for the bandwidth it's a big difference. This distinction between 
metadata and representation is not unique to S3, and the solution is general to all 
resource-oriented web services. The HEAD method gives any client a way of fetching 
the metadata for any resource, without also fetching its (possibly enormous) 
representation. 

Of course, sometimes you do want to download the movie, and for that you need a 
GET request. I've put the GET request in the accessor method S3: :0bject#value, in 
Example 3-11. Its structure mirrors that of S3: :Object#metadata. 

Example 3-11. S3 Ruby client: the S3::Object#value method 

# Retrieves the value of this Object, possibly fetching it 

# (along with the metadata) from S3, 
def value 

# If there's no value yet... 
unless Rvalue 

# Make a GET request to this Object's URI. 
response = open(uri) 



62 | Chapter 3: What Makes RESTful Services Different? 



# Read the metadata from the HTTP headers in the response. 
storejnetadata(response.meta) unless |5)metadata 

# Read the value from the entity-body 
(Svalue = response. read 

end 

return Rvalue 
end 

The client stores objects on the S3 service the same way it stores buckets: by sending a 
PUT request to a certain URL The bucket PUT is trivial because a bucket has no dis- 
tinguishing features other than its name, which goes into the URI of the PUT request. 
An object PUT is more complex. This is where the HTTP client specifies an object's 
metadata (such as Content -Type) and value. This information will be made available on 
future HEAD and GET requests. 

Fortunately, setting up the PUT request is not terribly complicated, because an object's 
value is whatever the client says it is. I don't have to wrap the object's value in an XML 
document or anything. I just send the data as is, and set HTTP headers that correspond 
to the items of metadata in my metadata hash (see Example 3-12). 

Example 3-12. S3 Ruby client: the S3::Object#put method 

# Store this Object on S3, 
def put(acl_policy=nil) 

# Start from a copy of the original metadata, or an empty hash if 

# there is no metadata yet. 

args = ^metadata ? ^metadata. clone : {} 

# Set the HTTP method, the entity-body, and some additional HTTP 

# headers. 

args [: method] = :put 

args["x-amz-acl"] = acl_policy if acl_policy 
if Rvalue 

args["Content-Length"] = Rvalue. size. to_s 
args[:body] = Rvalue 
end 

# Make a PUT request to this Object's URI. 
open(uri, args) 

return self 
end 

The S3: :Object#delete implementation (see Example 3-13) is identical to 
S3: :Bucket#delete. 

Example 3-13. S3 Ruby client: the S3::Object#delete method 

# Deletes this Object, 
def delete 

# Make a DELETE request to this Object's URI. 
open(uri, :method => :delete) 

end 



An S3 Client | 63 



And Example 3- 14 shows the method for turning HTTP response headers into S3 object 
metadata. Except for Content-Type, you should prefix all the metadata headers you set 
with the string "x-amz-meta-". Otherwise they won't make the round trip to the S3 
server and back to a web service client. S3 will think they're quirks of your client soft- 
ware and discard them. 

Example 3-14. S3 Ruby client: the S3::Object#store_metadata method 
private 

# Given a hash of headers from a HTTP response, picks out the 

# headers that are relevant to an S3 Object, and stores them in the 

# instance variable ^metadata, 
def store_metadata(new_metadata) 

^metadata = {} 
new_metadata.each do |h,v| 
if RE LEVANT_HEADERS. member ?(h) || h.index( 'x-amz-meta ' ) == 

|Smetadata[h] = v 
end 
end 
end 

RELEVANT_HEADERS = ['content-type', 'content-disposition', 'content-range', 
'x-amz-missing-meta' ] 

end 

Request Signing and Access Control 

I've put it off as long as I can, and now it's time to deal with S3 authentication. If your 
main interest is in RESTful services in general, feel free to skip ahead to the section on 
using the S3 library in clients . But if the inner workings of S3 have piqued your interest, 
read on. 

The code I've shown you so far makes HTTP requests all right, but S3 rejects them, 
because they don't contain the all-important Authorization header. S3 has no proof 
that you're the owner of your own buckets. Remember, Amazon charges you for the 
data stored on their servers and the bandwidth used in transferring that data. If S3 
accepted requests to your buckets with no authorization, anyone could store data in 
your buckets and you'd get charged for it. 

Most web services that require authentication use a standard HTTP mechanism to 
make sure you are who you claim to be. But S3's needs are more complicated. With 
most web services you never want anyone else using your data. But one of the uses of 
S3 is as a hosting service. You might want to host a big movie file on S3, let anyone 
download it with their BitTorrent client, and have Amazon send you the bill. 

Or you might be selling access to movie files stored on S3. Your e-commerce site takes 
payment from a customer and gives them an S3 URI they can use to download the 
movie. You're delegating to someone else the right to make a particular web service 
call (a GET request) as you, and have it charged to your account. 



64 | Chapter 3: What Makes RESTful Services Different? 



The standard mechanisms for HTTP authentication can't provide security for that kind 
of application. Normally, the person who's sending the HTTP request needs to know 
the actual password. You can prevent someone from spying on your password, but you 
can't say to someone else: "here's my password, but you must promise only to use it 
to request this one URI." 

This is a job for public-key cryptography. Every time you make an S3 request, you use 
your "private" key (remember, not truly private: Amazon knows it too) to sign the 
important parts of the request. That'd be the URI, the HTTP method you're using, and 
a few of the HTTP headers. Only someone with the "private" key can create these 
signatures for your requests, which is how Amazon knows it's okay to charge you for 
the request. But once you've signed a request, you can send the signature to a third 
party without revealing your "private" key. The third party is then free to send an 
identical HTTP request to the one you signed, and have Amazon charge you for it. In 
short: someone else can make a specific request as you, for a limited time, without 
having to know your "private" key. 

There is a simpler way to give anonymous access to your S3 objects, and I discuss it 
below. But there's no way around signing your own requests, so even a simple library 
like this one must support request signing if it's going to work. I'm reopening the 
S3: : Authorized Ruby module now. I'm going to give it the ability to intercept calls to 
the open method, and sign HTTP requests before they're made. Since S3 : : Bucket List, 
S3: : Bucket, and S3: : Object have all included this module, they'll inherit this ability as 
soon as I define it. Without the code I'm about to write, all those open calls I defined 
in the classes above will send unsigned HTTP requests that just bounce off S3 with 
response code 403 ("Forbidden"). With this code, you'll be able to generate signed 
HTTP requests that pass through S3's security measures (and cost you money). The 
code in Example 3-15 and the other examples that follow is heavily based on Amazon's 
own example S3 library. 

Example 3-15. S3 Ruby client: the S3 "Authorized module 

module Authorized 

# These are the standard HTTP headers that S3 considers interesting 

# for purposes of request signing. 

INTERESTING_HEADERS = ['content-type', 'content-md5' , 'date'] 

# This is the prefix for custom metadata headers. All such headers 

# are considered interesting for purposes of request signing. 
AMAZON_HEADER_PREFIX = 'x-amz-' 

# An S3-specific wrapper for rest-open-uri's implementation of 

# open(). This implementation sets some HTTP headers before making 

# the request. Most important of these is the Authorization header, 

# which contains the information Amazon will use to decide who to 

# charge for this request. 

def open(uri, headers_and_options={}, *args, Sblock) 
headers_and_options = headers_and_options.dup 
headers_and_options[ 'Date' ] | |= Time. now. httpdate 
headers_and_options[ 'Content-Type' ] ||= " 



Request Signing and Access Control | 65 



signed = signature(uri, headers_and_options[ :method] || :get, 

headers_and_options) 
headers_and_options[ 'Authorization' ] = "AWS #{@@public_key}:#{signed}" 
Kernel: :open(uri, headers_and_options, *args, &block) 
end 

The tough work here is in the signature method, not yet defined. This method needs 
to construct an encrypted string to go into a request's Authorization header: a string 
that convinces the S3 service that it's really you sending the request — or that you've 
authorized someone else to make the request at your expense (see Example 3-16). 

Example 3-16. S3 Ruby client: the Authorized#signature module 

# Builds the cryptographic signature for an HTTP request. This is 

# the signature (signed with your private key) of a "canonical 

# string" containing all interesting information about the request, 
def signature(uri, method=:get, headers={}, expires=nil) 

# Accept the URI either as a string, or as a Ruby URI object, 
if uri.respond_to? :path 

path = uri.path 
else 

uri = URI.parse(uri) 

path = uri.path + (uri. query ? "?" + query : "") 
end 

# Build the canonical string, then sign it. 

signed_string = sign(canonical_string(method, path, headers, expires)) 
end 

Well, this method passes the buck again, by calling sign on the result of 
canonical_string. Let's look at those two methods, starting with canonical_string. It 
turns an HTTP request into a string that looks something like Example 3-17. That string 
contains everything interesting (from S3's point of view) about an HTTP request, in a 
specific format. The interesting data is the HTTP method (PUT), the Content-type 
("text/plain"), a date, a few other HTTP headers ("x-amz-metadata"), and the path 
portion of the URI ("/crummy. com/myobject"). This is the string that sign will sign. 
Anyone can create this string, but only the S3 account holder and Amazon know how 
to produce the correct signature. 

Example 3-1 7. The canonical string for a sample request 
PUT 

text/plain 

Fri, 27 Oct 2006 21:22:41 GMT 

x-amz-metadata:Here's some metadata for the myobject object, 
/crummy . com/myobject 

When Amazon's server receives your HTTP request, it generates the canonical string, 
signs it (again, Amazon knows your secret key), and sees whether the two signatures 
match. That's how S3 authentication works. If the signatures match, your request goes 
through. Otherwise, you get a response code of 403 ("Forbidden"). 



66 | Chapter 3: What Makes RESTful Services Different? 



Example 3-18 shows the code to generate the canonical string. 

Example 3-18. S3 Ruby client: the Authorized# canonical _string method 

# Turns the elements of an HTTP request into a string that can be 

# signed to prove a request comes from your web service account, 
def canonical_string(method, path, headers, expires=nil) 

# Start out with default values for all the interesting headers, 
signjieaders = {} 

INTERESTING_HEADERS.each { |header| sign_headers[header] = " } 

# Copy in any actual values, including values for custom S3 

# headers. 

headers. each do | header, value| 
if header. respond_to? :to_str 
header = header. downcase 

# If it's a custom header, or one Amazon thinks is interesting... 
if I NTERESTING_HEADERS. member? (header) | | 
header. index (AMAZON_HEADER_PRE FIX) == 
# Add it to the header has. 
sign_headers[header] = value. to_s. strip 
end 
end 
end 

# This library eliminates the need for the x-amz-date header that 

# Amazon defines, but someone might set it anyway. If they do, 

# we'll do without HTTP's standard Date header. 
sign_headers[ 'date' ] = ' ' if signjieaders. has_key? 'x-amz-date' 

# If an expiration time was provided, it overrides any Date 

# header. This signature will be valid until the expiration time, 

# not only during the single second designated by the Date header, 
signjieaders [ 'date' ] = expires. to_s if expires 

# Now we start building the canonical string for this request. We 

# start with the HTTP method, 
canonical = method. to_s.upcase + "\n" 

# Sort the headers by name, and append them (or just their values) 

# to the string to be signed. 

sign Jeaders. sort Jy { |h| h[o] }.each do |header, value| 
canonical « header « ":" if header. index ( AMAZON JIEADERJRE FIX) == 
canonical << value << "\n" 

end 

# The final part of the string to be signed is the URI path. We 

# strip off the query string, and (if necessary) tack one of the 

# special S3 query parameters back on: 'acl', 'torrent', or 

# 'logging'. 

canonical « path.gsub(/\?.*$/, ") 

for param in ['acl', 'torrent', 'logging'] 
if path =~ Regexp.new("[S?]#{param}($|&|=)") 
canonical << "?" << param 



Request Signing and Access Control | 67 



break 
end 
end 

return canonical 
end 

The implementation of sign is just a bit of plumbing around Ruby's standard crypto- 
graphic and encoding interfaces (see Example 3-19). 

Example 3-19. S3 Ruby client: the Authorized#sign method 

# Signs a string with the client's secret access key, and encodes the 

# resulting binary string into plain ASCII with base64. 
def sign(str) 

digest_generator = OpenSSL: :Digest: :Digest.new( ' shal' ) 
digest = OpenSSL: :HMAC.digest(digest_generator, l«)@private_key, str) 
return Base64.encode64(digest). strip 
end 

Signing a URI 

My S3 library has one feature still to be implemented. I've mentioned a few times that 
S3 lets you sign an HTTP request and give the URI to someone else, letting them make 
that request as you. Here's the method that lets you do this: signed_uri (see Exam- 
ple 3-20). Instead of making an HTTP request with open, you pass the open arguments 
into this method, and it gives you a signed URI that anyone can use as you. To limit 
abuse, a signed URI works only for a limited time. You can customize that time by 
passing a Time object in as the keyword argument : expires. 

Example 3-20. S3 Ruby client: the Authorized#signed_uri method 

# Given information about an HTTP request, returns a URI you can 

# give to anyone else, to let them them make that particular HTTP 

# request as you. The URI will be valid for 15 minutes, or until the 

# Time passed in as the : expires option, 
def signed_uri(headers_and_options={}) 

expires = headers_and_options[ : expires] || (Time. now. to_i + (15 * 60)) 
expires = expires. to_i if expires. respond_to? :to_i 
headers_and_options . delete ( : expires) 

signature = URI.escape(signature(uri, headers_and_options[ : method ] , 

headers_and_options, nil)) 

q = (uri.index("?")) ? "&" : "?" 

"#{uri}#{q}Signature=#{signature}&Expires=#{expires}SAWSAccessKeyId=#{@@public_key}" 
end 
end 

end # Remember the all-encompassing S3 module? This is the end. 

Here's how it works. Suppose I want to give a customer access to my hosted file at 
https://s3.amazonaws.com/BobProductions/KomodoDragon.avi. I can run the code in 
Example 3-21 to generate a URI for my customer. 



68 | Chapter 3: What Makes RESTful Services Different? 



Example 3-21. Generating a signed URI 

#!/usr/bin/rubyl.9 

# s3-signed-uri.rb 
require 'S3lib' 

bucket = S3: :Bucket.new("BobProductions") 

object = S3: :Object.new(bucket, "KomodoDragon.avi") 

puts object. signed_uri 

# "https : //s3 . amazonaws . com/BobProductions/KomodoDragon . avi 

# ?Signature=D%2Fu6kxT3jOzHaFXjsLbowgpzExQ%3D 

# &Expires=ll62156499&AWSAccessKeyId=OF9DBXKB5274]KT]8DG2" 

That URI will be valid for 15 minutes, the default for my signed_uri implementation. 
It incorporates my public key (AWSAccessKeyld), the expiration time (Expires), and the 
cryptographic Signature. My customer can visit this URI and download the movie file 
KomodoDragon.avi. Amazon will charge me for my customer's use of their bandwidth. 
If my customer modifies any part of the URI (maybe they to try to download a second 
movie too), the S3 service will reject their request. An untrustworthy customer can send 
the URI to all of their friends, but it will stop working in 15 minutes. 

You may have noticed a problem here. The canonical string usually includes the value 
of the Date header. When my customer visits the URI you signed, their web browser 
will surely send a different value for the Date header. That's why, when you're gener- 
ating a canonical string to give to someone else, you set an expiration date instead of a 
request date. Look back to Example 3-18 and the implementation of 
canonical_string, where the expiration date (if provided) overwrites any value for the 
Date header. 

Setting Access Policy 

What if I want to make an object publicly accessible? I want to serve my files to the 
world and let Amazon deal with the headaches of server management. Well, I could 
set an expiration date very far in the future, and give out the enormous signed URI to 
everyone. But there's an easier way to get the same results: allow anonymous access. 
You can do this by setting the access policy for a bucket or object, telling S3 to respond 
to unsigned requests for it. You do this by sending the x-amz-acl header along with the 
PUT request that creates the bucket or object. 

That's what the acl_policy argument to Bucket#put and Object#put does. If you want 
to make a bucket or object publicly readable or writable, you pass an appropriate value 
in for acl_policy . My client sends that value as part of the custom HTTP request header 
X-amz-acl. Amazon S3 reads this request header and sets the rules for bucket or object 
access appropriately. 

The client in Example 3-22 creates an S3 object that anyone can read by visiting its URI 
at https://s3.amazonaws.com/BobProductions/KomodoDragon-Trailer.avi. In this sce- 
nario, I'm not selling my movies: just using Amazon as a hosting service so I don't have 
to serve movies from my own web site. 



Request Signing and Access Control | 69 



Example 3-22. Creating a publicly-readable object 

#!/usr/bin/ruby -w 
# s3-public-object.rb 
require 'S3lib' 

bucket = S3: : Bucket. new("BobProductions") 

object = S3: :Object.new(bucket, "KomodoDragon-Trailer.avi") 

object. put ("public-read") 

S3 understands four access policies: 
private 

The default. Only requests signed by your "private" key are accepted, 
public-read 

Unsigned GET requests are accepted: anyone can download an object or list a 
bucket. 

public-write 

Unsigned GET and PUT requests are accepted. Anyone can modify an object, or 
add objects to a bucket. 

authenticated-read 

Unsigned requests are rejected, but read requests can be signed by the "private" 
key of any S3 user, not just your own. Basically, anyone with an S3 account can 
download your object or list your bucket. 

There are also fine-grained ways of granting access to a bucket or object, which I won't 
cover. If you're interested, see the section "Setting Access Policy with REST" in the S3 
technical documentation. That section reveals a parallel universe of extra resources. 
Every bucket /{name-of-bucket} has a shadow resource /{name-of-bucket}?acl corre- 
sponding to that bucket's access control rules, and every object /{name-of-bucket}/ 
{name-of-object} has a shadow ACL resource /{name-of-bucket}/{name-of-object}? 
acl. By sending PUT requests to these URIs, and including XML representations of 
access control lists in the request entity-bodies, you can set specific permissions and 
limit access to particular S3 users. 

Using the S3 Client Library 

I've now shown you a Ruby client library that can access just about the full capabilities 
of Amazon's S3 service. Of course, a library is useless without clients that use it. In the 
previous section I showed you a couple of small clients to demonstrate points about 
security, but now I'd like to show something a little more substantial. 

Example 3-23 is a simple command-line S3 client that can create a bucket and an object, 
then list the contents of the bucket. This client should give you a high-level picture of 
how S3's resources work together. I've annotated the lines of code that trigger HTTP 
requests, by describing the HTTP requests in comments off to the right. 



70 | Chapter 3: What Makes RESTful Services Different? 



Example 3-23. A sample S3 client 

#!/usr/bin/ruby -w 

# s3-sample-client.rb 
require 'S3lib' 

# Gather command-line arguments 
bucket_name, object_name, object_value = ARGV 
unless bucket_name 

puts "Usage: #{$0} [bucket name] [object name] [object value]" 
exit 
end 

# Find or create the bucket. 

buckets = S3: :BucketList.new.get # GET / 

bucket = buckets. detect { |b| b.name == bucket_name } 
if bucket 

puts "Found bucket #{bucket_name}. " 
else 

puts "Could not find bucket #{bucket_name}, creating it." 
bucket = S3: : Bucket. new(bucket_name) 

bucket. put # PUT /{bucket} 

end 

# Create the object. 

object = S3: :Object.new(bucket, object_name) 
object. metadata[ 'content-type' ] = 'text/plain' 
object. value = object_value 

object. put # PUT /{bucket}/{object} 

# For each object in the bucket... 

bucket. get[0]. each do |o| # GET /{bucket} 

# ...print out information about the object, 
puts "Name: #{o.name}" 

puts "Value: #{o. value}" # GET /{bucket }/{object} 

puts "Metadata hash: #{o. metadata. inspect}" 
puts 
end 



Clients Made Transparent with ActiveResource 

Since all RESTful web services expose basically the same simple interface, it's not a big 
chore to write a custom client for every web service. It is a little wasteful, though, and 
there are two alternatives. You can describe a service with a WADL file (introduced in 
the previous chapter, and covered in more detail in Chapter 9), and then access it with 
a generic WADL client. There's also a Ruby library called ActiveResource that makes 
it trivial to write clients for certain kinds of web services. 

ActiveResource is designed to run against web services that expose the rows and tables 
of a relational database. WADL can describe almost any kind of web service, but Ac- 
tiveResource only works as a client for web services that follow certain conventions. 
Right now, Ruby on Rails is the only framework that follows the conventions. But any 



Clients Made Transparent with ActiveResource | 71 



web service can answer requests from an ActiveResource client: it just has to expose 
its database through the same RESTful interface as Rails. 

As of the time of writing, there are few publicly available web services that can be used 
with an ActiveResource client (I list a couple in Appendix A). To show you an example 
I'm going create a small Rails web service of my own. I'll be able to drive my service 
with an ActiveResource client, without writing any HTTP client or XML parsing code. 

Creating a Simple Service 

My web service will be a simple notebook: a way of keeping timestamped notes to 
myself. I've got Rails 1.2 installed on my computer, so I can create the notebook service 
like this: 

$ rails notebook 
$ cd notebook 

I create a database on my system called notebook_development, and edit the Rails file 
notebook/ 'config/database.yml to give Rails the information it needs to connect to my 
database. Any general guide to Rails will have more detail on these initial steps. 

Now I've created a Rails application, but it doesn't do anything. I'm going to generate 
code for a simple, RESTful web service with the scaff old_resource generator. I want 
my notes to contain a timestamp and a body of text, so I run the following command: 

$ ruby script/generate scaff old_resource note date: date body: text 

create app/views/notes 

create app/views/notes/index.rhtml 

create app/views/notes/show. rhtml 

create app/views/notes/new.rhtml 

create app/views/notes/edit . rhtml 

create app/views/layouts/ notes . rhtml 

create public/stylesheets/scaffold. ess 

create app/models/note.rb 

create app/controllers/notes_controller . rb 

create test/f unctional/notes_controller_test . rb 

create app/helpers/ notes_helper . rb 

create test/unit/note_test.rb 

create test/f ixtures/notes.yml 

create db/migrate 

create db/ migrate/001_create_notes . rb 

route map. resources : notes 

Rails has generated a complete set of web service code — model, view, and controller 
— for my "note" object. There's code in db/migrate/001_create_notes.rb that creates 
a database table called notes with three fields: a unique ID, a date (date), and a piece 
of text (body). 

The model code in app/models/note.rb provides an ActiveResource interface to the 
database table. The controller code in app/controllers/notes_controller.rb exposes 
that interface to the world through HTTP, and the views in app/views/notes define the 



72 | Chapter 3: What Makes RESTful Services Different? 



Listing notes 

Date Body 

2006-06-05 What if I wrote a book about REST? Show Edit Destroy 
2006-1 2-1 6 Pasta for lunch maybe? Show Edit Destroy 

Mew note 



Figure 3-1. The notebook web application with a few entered notes 

user interface. It adds up to a RESTful web service — not a very fancy one, but one that's 
good enough for a demo or to use as a starting point. 

Before starting the service I need to initialize the database: 
$ rake dbrmigrate 

== CreateNotes: migrating ===================================================== 

-- create_table(: notes) 
-> 0.0119s 

== CreateNotes: migrated (o. 0142s) ============================================ 

Now I can start the notebook application and start using my service: 

$ script/server 

=> Booting WEBrick... 

=> Rails application started on http://o. 0.0. 0:3000 

=> Ctrl-C to shutdown server; call with --help for options 

An ActiveResource Client 

The application I just generated is not much use except as a demo, but it demos some 
pretty impressive features. First, it's both a web service and a web application. I can 
visit http://localhost-.3000/notes in my web browser and create notes through the web 
interface. After a while the view of http://localhost-.3000/notes might look like 
Figure 3-1. 

If you've ever written a Rails application or seen a Rails demo, this should look familiar. 
But in Rails 1.2, the generated model and controller can also act as a RESTful web 
service. A programmed client can access it as easily as a web browser can. 

Unfortunately, the ActiveResource client itself was not released along with Rails 1.2. 
As of the time of writing, it's still being developed on the tip of the Rails development 
tree. To get the code I need to check it out from the Subversion version control 
repository: 

$ svn co http://dev.rubyonrails.org/svn/rails/trunk activeresource_client 
$ cd activeresource_client 



Clients Made Transparent with ActiveResource | 73 



Now I'm ready to write ActiveResource clients for the notebook's web service. Exam- 
ple 3-24 is a client that creates a note, modifies it, lists the existing notes, and then 
deletes the note it just created. 

Example 3-24. An ActiveResource client for the notebook service 
#!/usr/bin/ruby -w 

# activeresource- notebook-manipulation. rb 

require ' activesupport/lib/active_support ' 
require ' activeresource/lib/active_resource ' 

# Define a model for the objects exposed by the site 
class Note < ActiveResource: : Base 

self. site = 'http://localhost:3000/' 
end 

def show_notes 

notes = Note. find :all # GET /notes. xml 

puts "I see #{ notes. size} note(s):" 

notes. each do | note | 
puts " #{note.date}: #{note.body}" 

end 
end 

newjiote = Note. new(: date => Time. now, :body => "A test note") 
new_note.save # POST /notes. xml 

new_note.body = "This note has been modified." 

new_note.save # PUT /notes/{id}.xml 

show_notes 

new_note. destroy # DELETE /notes/{id}.xml 

puts 

show_notes 

Example 3-25 shows the output when I run that program: 

Example 3-25. A run of activeresource-notebook-manipulation.rb 
I see 3 note(s) : 

2006-06-05: What if I wrote a book about REST? 
2006-12-18: Pasta for lunch maybe? 
2006-12-18: This note has been modified. 

I see 2 note(s) : 

2006-06-05: What if I wrote a book about REST? 
2006-12-18: Pasta for lunch maybe? 

If you're familiar with ActiveRecord, the object-relational mapper that connects Rails 
to a database, you'll notice that the ActiveResource interface looks almost exactly the 
same. Both libraries provide an object-oriented interface to a wide variety of objects, 
each of which exposes a uniform interface. With ActiveRecord, the objects live in a 



74 | Chapter 3: What Makes RESTful Services Different? 



database and are exposed through SQL, with its SELECT, INSERT, UPDATE, and 
DELETE. With ActiveResource, they live in a Rails application and are exposed through 
HTTP, with its GET, POST, PUT, and DELETE. 

Example 3-26 is an excerpt from the Rails server logs at the time I ran my ActiveRe- 
source client. The GET, POST, PUT, and DELETE requests correspond to the com- 
mented lines of code back in Example 3-24. 

Example 3-26. The HTTP requests made by activeresource-notebook-manipulation.rb 

"POST /notes. xml HTTP/l.l" 201 
"PUT /notes/5. xml HTTP/l.l" 200 
"GET /notes. xml HTTP/l.l" 200 
"DELETE /notes/5. xml HTTP/l.l" 200 
"GET /notes. xml HTTP/l.l" 200 

What's going on in these requests? The same thing that's going on in requests to S3: 
resource access through HTTP's uniform interface. My notebook service exposes two 
kinds of resources: 

• The list of notes (/notes . xml) . Compare to an S3 bucket, which is a list of objects. 

• A note (/notes/{id}.xml). Compare to an S3 object. 

These resources expose GET, PUT, and DELETE, just like the S3 resources do. The 
list of notes also supports POST to create a new note. That's a little different from S3, 
where objects are created with PUT, but it's just as RESTful. 

When the client runs, XML documents are transferred invisibly between client and 
server. They look like the documents in Example 3-27 or 3-28: simple depictions of the 
underlying database rows. 

Example 3-27. The response entity-body from a GET request to Inotes.xml 

<?xml version="l.0" encoding="UTF-8"?> 
<notes> 
<note> 

<body>What if I wrote a book about REST?</body> 

<date type="date">2006-06-05</date> 

<id type="integer">2</id> 
</note> 
<note> 

<body>Pasta for lunch maybe?</body> 
<date type="date">2006-12-l8</date> 
<id type="integer">3</id> 
</note> 
</notes> 

Example 3-28. A request entity-body sent as part of a PUT request to lnotes/5.xml 

<?xml version="l.0" encoding="UTF-8"?> 
<note> 

<body>This note has been modified. </body> 
</note> 



Clients Made Transparent with ActiveResource | 75 



A Python Client for the Simple Service 

Right now the only ActiveResource client library is the Ruby library, and Rails is the 
only framework that exposes ActiveResource-compatible services. But nothing's hap- 
pening here except HTTP requests that pass XML documents into certain URIs and 
get XML documents back. There's no reason why a client in some other language 
couldn't send those XML documents, or why some other framework couldn't expose 
the same URIs. 

Example 3-29 is a Python implementation of the client program from Example 3-24. 
It's longer than the Ruby program, because it can't rely on ActiveResource. It has to 
build its own XML documents and make its own HTTP requests, but its structure is 
almost exactly the same. 

Example 3-29. A Python client for an ActiveResource service 
#!/usr/bin/python 

# activeresource-notebook-manipulation.py 

from elementtree.ElementTree import Element, SubElement, tostring 
from elementtree import ElementTree 
import httplib2 
import time 

BASE = "http://localhost:3000/" 
client = httplib2.Http(". cache") 

def showNotesQ : 

headers, xml = client. request(BASE + "notes. xml") 
doc = ElementTree. fromstring(xml) 
for note in doc. findall( ' note' ) : 

print "%s: %s" % (note. find( 'date' ) .text, note. find( ' body ') .text) 

newNote = Element("note") 

date = SubElement(newNote, "date") 

date. attrib[ 'type' ] = "date" 

date. text = time.strftime("%Y-%m-%d", time.localtime()) 
body = SubElement(newNote, "body") 
body. text = "A test note" 

headers, ignore = client. request(BASE + "notes. xml", "POST", 

body= tostring(newNote), 

headers={' content-type' : 'application/xml' }) 

newlIRI = headers [' location' ] 

modifiedBody = Element("note") 

body = SubElement(modifiedBody, "body") 

body. text = "This note has been modified" 

client. request(newURI, "PUT", 

body=tostring(modifiedBody) , 

headers={' content-type' : 'application/xml'}) 

showNotesQ 



76 | Chapter 3: What Makes RESTful Services Different? 



client. request (newURI, "DELETE") 
print 

showNotesQ 

Parting Words 

Because RESTful web services have simple and well-defined interfaces, it's not difficult 
to clone them or swap out one implementation for another. Park Place (http://code.why 
theluckystiff.net/parkplace) is a Ruby application that exposes the same HTTP interface 
as S3. You can use Park Place to host your own version of S3. S3 libraries and client 
programs will work against your Park Place server just as they now do against https:// 
s3 . amazonaws. com/. 

It's also possible to clone ActiveResource. No one has done this yet, but it shouldn't 
be difficult to write a general ActiveResource client for Python or any other dynamic 
language. In the meantime, writing a one-off client for an ActiveResource-compatible 
service is no more difficult than writing a client for any other RESTful service. 

By now you should feel comfortable with the prospect of writing a client for any REST- 
ful or REST-RPC hybrid service, whether it serves XML, HTML, JSON, or some mix- 
ture. It's all just HTTP requests and document parsing. 

You should also be getting a feel for what differentiates RESTful web services like S3 
and Yahool's search services from RPC-style and hybrid services like the Flickr and 
del.icio.us APIs. This is not a judgement about the service's content, only about its 
architecture. In woodworking it's important to work with the grain of the wood. The 
Web, too, has a grain, and a RESTful web service is one that works with it. 

In the coming chapters I'll show how you can create web services that are more like S3 
and less like the del.icio.us API. This culminates in Chapter 7, which reinvents 
del.icio.us as a RESTful web service. 



Parting Words | 77 



CHAPTER 4 



The Resource-Oriented Architecture 



I've shown you the power of REST, but I haven't shown you in any systematic way how 
that power is structured or how to expose it. In this chapter I outline a concrete RESTful 
architecture: the Resource-Oriented Architecture (ROA). The ROA is a way of turning 
a problem into a RESTful web service: an arrangement of URIs, HTTP, and XML that 
works like the rest of the Web, and that programmers will enjoy using. 

In Chapter 1 1 classified RESTful web services by their answers to two questions. These 
answers correspond to two of the four defining features of REST: 

• The scoping information ("why should the server send this data instead of that 
data?") is kept in the URI. This is the principle of addressability . 

• The method information ("why should the server send this data instead of deleting 
it?") is kept in the HTTP method. There are only a few HTTP methods, and ev- 
eryone knows ahead of time what they do. This is the principle of the uniform- 
interface. 

In this chapter I introduce the moving parts of the Resource-Oriented Architecture: 
resources (of course), their names, their representations, and the links between them. 
I explain and promote the properties of the ROA: addressability, statelessness, con- 
nectedness, and the uniform interface. I show how the web technologies (HTTP, URIs, 
and XML) implement the moving parts to make the properties possible. 

In the previous chapters I illustrated concepts by pointing to existing web services, like 
S3. 1 continue that tradition in this chapter, but I'll also illustrate concepts by pointing 
to existing web sites. Hopefully I've convinced you by now that web sites are web 
services, and that many web applications (such as search engines) are RESTful web 
services. When I talk about abstract concepts like addressability, it's useful to show 
you real URIs, which you can type into your web browser to see the concepts in action. 

Resource-Oriented What Now? 

Why come up with a new term, Resource-Oriented Architecture? Why not just say 
REST? Well, I do say REST, on the cover of this book, and I hold that everything in the 



79 



Resource-Oriented Architecture is also RESTful. But REST is not an architecture: it's 
a set of design criteria. You can say that one architecture meets those criteria better 
than another, but there is no one "REST architecture." 

Up to now, people have tended to mint one-off architectures as they design their serv- 
ices, according to their own understandings of REST. The most obvious outcome of 
this is the wide variety of REST-RPC hybrid web services that their creators claim are 
RESTful. I'm trying to put a stop to that by presenting a set of concrete rules for building 
web services that really will be RESTful. In the next two chapters I'll even show simple 
procedures you can follow to turn requirements into resources. If you don't like my 
rules, you'll at least have an idea of what you can change and stay RESTful. 

As a set of design criteria, REST is very general. In particular, it's not tied to the Web. 
Nothing about REST depends on the mechanics of HTTP or the structure of URIs. But 
I'm talking about web services, so I explicitly tie the Resource-Oriented Architecture 
to the technologies of the Web. I want to talk about how to do REST with HTTP and 
URIs, in specific programming languages. If the future produces RESTful architectures 
that don't run on top of the Web, their best practices will probably look similar to the 
ROA, but the details will be different. We'll cross that bridge when we come to it. 

The traditional definition of REST leaves a lot of open space, which practitioners have 
seeded with folklore. I deliberately go further than Roy Fielding in his dissertation, or 
the W3C in their standards: I want to clear some of that open space so that the folklore 
has room to grow into a well-defined set of best practices. Even if REST were an ar- 
chitecture, it wouldn't be fair to call my architecture by the same name. I'd be tying my 
empirical observations and suggestions to the more general thoughts of those who built 
the Web. 

My final reason for coming up with a new term is that "REST" is a term used in religious 
nerd wars. When it's used, the implication is usually that there is one true RESTful 
architecture and it's the one the speaker prefers. People who prefer another RESTful 
architecture disagree. The REST community fragments, despite a general agreement 
on basic things like the value of URIs and HTTP. 

Ideally there would be no religious wars, but I've seen enough to know that wishing 
won't end them. So I'm giving a distinctive name to my philosophy of how RESTful 
applications should be designed. When these ideas are, inevitably, used as fodder in 
wars, people who disagree with me can address aspects of the Resource-Oriented Ar- 
chitecture separate from other RESTful architectures, and from REST in general. Clarity 
is the first step toward understanding. 

The phrases "resource-oriented" and "resource-oriented architecture" have been used 
to describe RESTful architectures in general. T don't claim that "Resource-Oriented 
Architecture" is a completely original term, but I think that my usage meshes well with 
preexisting uses, and that it's better to use this term than claim to speak for REST as a 
whole. 



80 | Chapter 4: The Resource-Oriented Architecture 



What's a Resource? 



A resource is anything that's important enough to be referenced as a thing in itself. If 
your users might "want to create a hypertext link to it, make or refute assertions about 
it, retrieve or cache a representation of it, include all or part of it by reference into 
another representation, annotate it, or perform other operations on it", then you should 
make it a resource." 1 " 

Usually, a resource is something that can be stored on a computer and represented as 
a stream of bits: a document, a row in a database, or the result of running an algorithm. 
A resource may be a physical object like an apple, or an abstract concept like courage, 
but (as we'll see later) the representations of such resources are bound to be 
disappointing. 

Here are some possible resources: 

• Version 1.0.3 of the software release 

• The latest version of the software release 

• The first weblog entry for October 24, 2006 

• A road map of Little Rock, Arkansas 

• Some information about jellyfish 

• A directory of resources pertaining to jellyfish 

• The next prime number after 1024 

• The next five prime numbers after 1024 

• The sales numbers for Q42004 

• The relationship between two acquaintances, Alice and Bob 

• A list of the open bugs in the bug database 



URIs 

What makes a resource a resource? It has to have at least one URI. The URI is the name 
and address of a resource. If a piece of information doesn't have a URI, it's not a resource 
and it's not really on the Web, except as a bit of data describing some other resource. 



The earliest instance of "resource-oriented" I've found is a 2004 IBM developerWorks article hy James Snell: 
"Resource-oriented vs. activity-oriented Web services" (http://www-128.ibm.com/developerworks/xml/ 
library '/ws-restvsoap/) . Alex Bunardzic used "Resource-Oriented Architecture" in August 2006, before this 
book was announced: http://jooto.com/blog/index.php/2006/08/08/replacing-service-oriented-architecture- 
with-resource-oriented-architecture/. I don't agree with everything in those articles, but I do acknowledge 
their priority in terminology. 

t "The Architecture of the World Wide Web" (http://www.w3.Org/TR/2004/REC-webarch-20041215/#p39), 
which is full of good quotes, incidentally: "Software developers should expect that sharing URIs across 
applications will be useful, even if that utility is not initially evident." This could be the battle cry of the ROA. 



What's a Resource? | 81 



Remember the sample session in the Preface, when I was making fun of HTTP 0.9? 
Let's say this is a HTTP 0.9 request for http://www.example.com/hello.txt: 

Client request Server response 

GET /hello. txt Hello, world! 



An HTTP client manipulates a resource by connecting to the server that hosts it (in this 
case, www.example.com), and sending the server a method ("GET") and a path to the 
resource ("/hello. txt"). Today's HTTP 1.1 is a little more complex than 0.9, but it works 
the same way. Both the server and the path come from the resource's URI. 



Client request 


Server response 


GET /hello. txt HTTP/l.l 


200 OK 


Host: www.example.com 


Content-Type: text/plain 




Hello, world! 



The principles behind URIs are well described by Tim Berners-Lee in Universal Re- 
source Identifiers — Axioms of Web Architecture (http://www.w3.org/Designhsues/Ax 
ioms). In this section I expound the principles behind constructing URIs and assigning 
them to resources. 

m\ 

* - I The URI is the fundamental technology of the Web. There were hyper- 
text systems before HTML, and Internet protocols before HTTP, but 
f A' they didn't talk to each other. The URI interconnected all these Internet 
protocols into a Web, the way TCP/IP interconnected networks like 
Usenet, Bitnet, and CompuServe into a single Internet. Then the Web 
co-opted those other protocols and killed them off, just like the Internet 
did with private networks. 

Today we surf the Web (not Gopher) , download files from the Web (not 
FTP sites), search publications from the Web (not WAIS), and have 
conversations on the Web (not Usenet newsgroups). Version control 
systems like Subversion and arch work over the Web, as opposed to the 
custom CVS protocol. Even email is slowly moving onto the Web. 

The web kills off other protocols because it has something most proto- 
cols lack: a simple way of labeling every available item. Every resource 
on the Web has at least one URI. You can stick a URI on a billboard. 
People can see that billboard, type that URI into their web browsers, 
and go right to the resource you wanted to show them. It may seem 
strange, but this everyday interaction was impossible before URIs were 
invented. 



82 | Chapter 4: The Resource-Oriented Architecture 



URIs Should Be Descriptive 

Here's the first point where the ROA builds upon the sparse recommendations of the 
REST thesis and the W3C recommendations. I propose that a resource and its URI 
ought to have an intuitive correspondence. Here are some good URIs for the resources 
I listed above: 

• http : //www. example . com/ software/ releases/l . . 3 . tar . gz 

• http : //www. example . com/ software/ releases/latest . tar . gz 

• http : //www. example . com/weblog/2006/10/24/0 

• http : / /www. example . com/map/roads/USA/AR/Little_Rock 

• http://www.example.com/wiki/Dellyfish 

• http : / /www. example . com/search/Dellyf ish 

• http : //www. example . com/ nextprime/1024 

• http : //www. example . com/ next-5-primes/l024 

• http : //www. example . com/ sales/2004/Q4 

• http : //www. example . com/ relationships/Alice; Bob 

• http://www.example.com/bugs/by-state/open 

URIs should have a structure. They should vary in predictable ways: you should not 
go to /search/Jellyfish for jellyfish and /i-want-to-know-about/Mice for mice. If a 
client knows the structure of the service's URIs, it can create its own entry points into 
the service. This makes it easy for clients to use your service in ways you didn't think of. 

This is not an absolute rule of REST, as we'll see in the "Name the Resources" section. 
URIs do not technically have to have any structure or predictability, but I think they 
should. This is one of the rules of good web design, and it shows up in RESTful and 
REST-RPC hybrid services alike. 

The Relationship Between URIs and Resources 

Let's consider some edge cases. Can two resources be the same? Can two URIs designate 
the same resource? Can a single URI designate two resources? 

By definition, no two resources can be the same. If they were the same, you'd only have 
one resource. However, at some moment in time two different resources may point to 
the same data. If the current software release is 1.0.3, then http://www.example.com/ 
software/releases/ 1.0 3. tar. gz and http://www.example.com/software/releases/lat- 
est.tar.gz will refer to the same file for a while. But the ideas behind those two URIs 
are different: one of them always points to a particular version, and the other points to 
whatever version is newest at the time the client accesses it. That's two concepts and 
two resources. You wouldn't link to latest when reporting a bug in version 1.0.3. 



URIs | 83 



A resource may have one URI or many. The sales numbers available at http://www.ex- 
ample.com/sales/2004/Q4 might also be available at http://www.example.com/sales/ 
Q42004. If a resource has multiple URIs, it's easier for clients to refer to the resource. 
The downside is that each additional URI dilutes the value of all the others. Some clients 
use one URI, some use another, and there's no automatic way to verify that all the URIs 
refer to the same resource. 

0\ 

• > One way to get around this is to expose multiple URIs for the same 

* resource, but have one of them be the "canonical" URI for that resource. 
iJt; When a client requests the canonical URI, the server sends the appro- 
priate data along with response code of 200 ("OK"). When a client re- 
quests one of the other URIs, the server sends a response code 303 ("See 
Also") along with the canonical URI. The client can't see whether two 
URIs point to the same resource, but it can make two HEAD requests 
and see if one URI redirects to the other or if they both redirect to a third 
URI. 

Another way is to serve all the URIs as though they were the same, but 
give the "canonical" URI in the Content- Location response header 
whenever someone requests a non-canonical URI. 

Fetching sales/2004/Q4 might get you the same bytestream as fetching sales/Q42004, 
because they're different URIs for the same resource: "sales for the last quarter of 2004. " 
Fetching releases/l.0.3.tar.gz might give you the exact same bytestream as fetching 
releases/latest. tar. gz, but they're different resources because they represent differ- 
ent things: "version 1.0.3" and "the latest version." 

Every URI designates exactly one resource. If it designated more than one, it wouldn't 
be a Universal Resource Identifier. However, when you fetch a URI the server may send 
you information about multiple resources: the one you requested and other, related 
ones. When you fetch a web page, it usually conveys some information of its own, but 
it also has links to other web pages. When you retrieve an S3 bucket with an Amazon 
S3 client, you get a document that contains information about the bucket, and infor- 
mation about related resources: the objects in the bucket. 

Addressability 

Now that I've introduced resources and their URIs, I can go in depth into two of the 
features of the ROA: addressability and statelessness. 

An application is addressable if it exposes the interesting aspects of its data set as re- 
sources. Since resources are exposed through URIs, an addressable application exposes 
a URI for every piece of information it might conceivably serve. This is usually an infinite 
number of URIs. 



84 | Chapter 4: The Resource-Oriented Architecture 



From the end-user perspective, addressability is the most important aspect of any web 
site or application. Users are clever, and they'll overlook or work around almost any 
deficiency if the data is interesting enough, but it's very difficult to work around a lack 
of addressability. 

Consider a real URI that names a resource in the genre "directory of resources about 
jellyfish": http://www. google. com/search?q=jelly fish. That jellyfish search is just as 
much a real URI as http://www.google.com. If HTTP wasn't addressable, or if the Google 
search engine wasn't an addressable web application, I wouldn't be able to publish that 
URI in a book. I'd have to tell you: "Open a web connection to google.com, type 'jel- 
lyfish' in the search box, and click the 'Google Search' button." 

This isn't an academic worry. Until the mid-1990s, when ftp:// URIs 
became popular for describing files on FTP sites, people had to write 
things like: "Start an anonymous FTP session on ftp . example . com. Then 
change to directory pub/files/ and download file file, txt." URIs made 
FTP as addressable as HTTP. Now people just write: "Download ftp:// 
ftp.example.com/pub/files/file.txt.'''' The steps are the same, but now they 
can be carried out by machine. 

But HTTP and Google are both addressable, so I can print that URI in a book. You can 
read it and type it in. When you do, you end up where I was when I went through the 
Google web application. 

You can then bookmark that page and come back to it later. You can link to it on a 
web page of your own. You can email the URI to someone else. If HTTP wasn't ad- 
dressable, you'd have to download the whole page and send the HTML file as an at- 
tachment. 

To save bandwidth, you can set up an HTTP proxy cache on your local network. The 
first time someone requests http://www. google. com/search?q=jellyfish, the cache will 
save a local copy of the document. The next time someone hits that URI, the cache 
might serve the saved copy instead of downloading it again. These things are possible 
only if every page has a unique identifying string: an address. 

It's even possible to chain URIs: to use one URI as input to another one. You can use 
an external web service to validate a page's HTML, or to translate its text into another 
language. These web services expect a URI as input. If HTTP wasn't addressable, you'd 
have no way of telling them which resource you wanted them to operate on. 

Amazon's S3 service is addressable because every bucket and every object has its own 
URI, as does the bucket list. Buckets and objects that don't exist yet aren't yet resources, 
but they too have their own URIs: you can create a resource by sending a PUT request 
to its URI. 

The filesystem on your home computer is another addressable system. Command-line 
applications can take a path to a file and do strange things to it. The cells in a spreadsheet 



m i 




Addressability | 85 



are also addressable; you can plug the name of a cell into a formula, and the formula 
will use whatever value is currently in that cell. URIs are the file paths and cell addresses 
of the Web. 

Addressability is one of the best things about web applications. It makes it easy for 
clients to use web sites in ways the original designers never imagined. Following this 
one rule gives you and your users many of the benefits of REST. This is why REST-RPC 
services are so common: they combine addressability with the procedure-call program- 
ming model. It's why I gave resources top billing in the name of the Resource-Oriented 
Architecture: because resources are the kind of thing that's addressable. 

This seems natural, the way the Web should work. Unfortunately, many web applica- 
tions don't work this way. This is especially true of Ajax applications. As I show in 
Chapter 11, most Ajax applications are just clients for RESTful or hybrid web services. 
But when you use these clients as though they are web sites, you notice that they don't 
feel like web sites. 

No need to pick on the little guys; let's continue our tour of the Google properties by 
considering the Gmail online email service. From the end-user perspective, there is only 
one Gmail URI: https://mail.google.com/. Whatever you do, whatever pieces of infor- 
mation you retrieve from or upload to Gmail, you'll never see a different URI. The 
resource "email messages about jellyfish" isn't addressable, the way Google's "web 
pages about jellyfish" is.-t-Yet behind the scenes, as I show in Chapter 11, is a web site 
that is addressable. The list of email messages about jellyfish does have a URI: it's https:// 
mail. google. com/mail/?q=jellyfish&search=query&view=tl. The problem is, you're not 
the consumer of that web site. The web site is really a web service, and the real consumer 
is a JavaScript program running inside your web browser. §The Gmail web service is 
addressable, but the Gmail web application that uses it is not. 

Statelessness 

Addressability is one of the four main features of the ROA. The second is statelessness. 
I'll give you two definitions of statelessness: a somewhat general definition and a more 
practical definition geared toward the ROA. 

Statelessness means that every HTTP request happens in complete isolation. When the 
client makes an HTTP request, it includes all information necessary for the server to 
fulfill that request. The server never relies on information from previous requests. If 
that information was important, the client would have sent it again in this request. 



* Compare the Ajax interface against the more addressable version of Gmail you get by starting off at the URI 
https://mail.google.com/mail/?ui=html. If you use this plain HTML interface, the resource "email messages 
about jellyfish" is addressable. 

§ Other consumers of this web service include the libgmail library for Python (http://libgmail.sourceforge.net/). 



86 | Chapter 4: The Resource-Oriented Architecture 



More practically, consider statelessness in terms of addressability. Addressability says 
that every interesting piece of information the server can provide should be exposed as 
a resource, and given its own URI. Statelessness says that the possible states of the server 
are also resources, and should be given their own URIs. The client should not have to 
coax the server into a certain state to make it receptive to a certain request. 

On the human web, you often run into situations where your browser's back button 
doesn't work correctly, and you can't go back and forth in your browser history. Some- 
times this is because you performed an irrevocable action, like posting a weblog entry 
or buying a book, but often it's because you're at a web site that violates the principle 
of statelessness. Such a site expects you to make requests in a certain order: A, B, then 
C. It gets confused when you make request B a second time instead of moving on to 
request C. 

Let's take the search example again. A search engine is a web service with an infinite 
number of possible states: at least one for every string you might search for. Each state 
has its own URI. You can ask the service for a directory of resources about mice: http:// 
www. google. com/search ?q=mice. You can ask for a directory of resources about jelly- 
fish: http://www. google. com/search?q=jelly fish. If you're not comfortable creating a URI 
from scratch, you can ask the service for a form to fill out: http://www.google.com/. 

When you ask for a directory of resources about mice or jellyfish, you don't get the 
whole directory. You get a single page of the directory: a list of the 10 or so items the 
search engine considers the best matches for your query. To get more of the directory 
you must make more HTTP requests. The second and subsequent pages are distinct 
states of the application, and they need to have their own URIs: something like http:/ I 
www. google. com/search?q=jellyfish&start=10. As with any addressable resource, you 
can transmit that state of the application to someone else, cache it, or bookmark it and 
come back to it later. 

Figure 4-1 is a simple state diagram showing how an HTTP client might interact with 
four states of a search engine. 

This is a stateless application because every time the client makes a request, it ends up 
back where it started. Each request is totally disconnected from the others. The client 
can make requests for these resources any number of times, in any order. It can request 
page 2 of "mice" before requesting page 1 (or not request page 1 at all), and the server 
won't care. 

By way of contrast, Figure 4-2 shows the same states arranged statefully, with states 
leading sensibly into each other. Most desktop applications are designed this way. 

That's a lot better organized, and if HTTP were designed to allow stateful interaction, 
HTTP requests could be a lot simpler. When the client started a session with the search 
engine it could be automatically fed the search form. It wouldn't have to send any 
request data at all, because the first response would be predetermined. If the client was 
looking at the first 10 entries in the mice directory and wanted to see entries 11-20, it 



Statelessness | 87 




Figure 4-1. A stateless search engine 




Figure 4-2. A stateful search engine 



could just send a request that said "start=10". It wouldn't have to send /search? 
q=mice&start=10, repeating the intitial assertions: "I'm searching, and searching for 
mice in particular." 

FTP works this way: it has a notion of a "working directory" that stays constant over 
the course of a session unless you change it. You might log in to an FTP server, cd to a 
certain directory, and get a file from that directory. You can get another file from the 
same directory, without having to issue a second cd command. Why doesn't HTTP 
support this? 

State would make individual HTTP requests simpler, but it would make the HTTP 
protocol much more complicated. An FTP client is much more complicated than an 
HTTP client, precisely because the session state must be kept in sync between client 
and server. This is a complex task even over a reliable network, which the Internet is not. 

To eliminate state from a protocol is to eliminate a lot of failure conditions. The server 
never has to worry about the client timing out, because no interaction lasts longer than 
a single request. The server never loses track of "where" each client is in the application, 



88 | Chapter 4: The Resource-Oriented Architecture 



because the client sends all necessary information with each request. The client never 
ends up performing an action in the wrong "working directory" due to the server keep- 
ing some state around without telling the client. 

Statelessness also brings new features. It's easier to distribute a stateless application 
across load-balanced servers. Since no two requests depend on each other, they can be 
handled by two different servers that never coordinate with each other. Scaling up is 
as simple as plugging more servers into the load balancer. A stateless application is also 
easy to cache: a piece of software can decide whether or not to cache the result of an 
HTTP request just by looking at that one request. There's no nagging uncertainty that 
state from a previous request might affect the cacheability of this one. 

The client benefits from statelessness as well. A client can process the "mice" directory 
up to page 50, bookmark /search?q=mice&start=50, and come back a week later with- 
out having to grind through dozens of predecessor states. A URI that works when you're 
hours deep into an HTTP session will work the same way as the first URI sent in a new 
session. 

To make your service addressable you have to put in some work, dissect your applica- 
tion's data into sets of resources. HTTP is an intrinsically stateless protocol, so when 
you write web services, you get statelessness by default. You have to do something to 
break it. 

The most common way to break statelessness is to use your framework's version of 
HTTP sessions. The first time a user visits your site, he gets a unique string that identifies 
his session with the site. The string may be kept in a cookie, or the site may propagate 
a unique string through all the URIs it serves a particular client. Here's an session cookie 
being set by a Rails application: 

Set-Cookie : _session_id=clc934bbe6l68dcb904d21a7f 5644a2d ; path=/ 

This URI propagates the session ID in a PHP application: http://www.example.com/ 
forums?PHPSESSID=27314962133. 

The important thing is, that nonsensical hex or decimal number is not the state. It's a 
key into a data structure on the server, and the data structure contains the state. There's 
nothing unRESTful about stateful URIs: that's how the server communicates possible 
next states to the client. However, there is something unRESTful about cookies, as I 
discuss in "The Trouble with Cookies." To use a web browser analogy, cookies break 
a web service client's back button. 

Think of the query variable start=10 in a URI, embedded in an HTML page served by 
the Google search engine. That's the server sending a possible next state to the client. 

But those URIs need to contain the state, not just provide a key to state stored on the 
server. start=10 means something on its own, and PHPSESSID=27314962133 doesn't. 
RESTfulness requires that the state stay on the client side, and be transmitted to the 
server for every request that needs it. The server can nudge the client toward new states, 
by sending stateful links for the client to follow, but it can't keep any state of its own. 



Statelessness | 89 



Application State Versus Resource State 

When we talk about "statelessness," what counts as "state"? What's the difference 
between persistent data, the useful server-side data that makes us want to use web 
services in the first place, and this state we're trying to keep off the server? The Flickr 
web service lets you upload pictures to your account, and those pictures are stored on 
the server. It would be crazy to make the client send every one of its pictures along with 
every request to flickr.com, just to keep the server from having to store any state. That 
would defeat the whole point of the service. But what's the difference between this 
scenario, and state about the client's session, which I claim should be kept off the server? 

The problem is one of terminology. Statelessness implies there's only one kind of state 
and that the server should go without it. Actually, there are two kinds of state. From 
this point on in the book I'm going to distinguish between application state, which 
ought to live on the client, and resource state, which ought to live on the server. 

When you use a search engine, your current query and your current page are bits of 
client state. This state is different for every client. You might be on page 3 of the search 
results for "jellyfish," and I might be on page 1 of the search results for "mice." The 
page number and the query are different because we took different paths through the 
application. Our respective clients store different bits of application state. 

A web service only needs to care about your application state when you're actually 
making a request. The rest of the time, it doesn't even know you exist. This means that 
whenever a client makes a request, it must include all the application states the server 
will need to process it. The server might send back a page with links, telling the client 
about other requests it might want to make in the future, but then it can forget all about 
the client until the next request. That's what I mean when I say a web service should 
be "stateless." The client should be in charge of managing its own path through the 
application. 

Resource state is the same for every client, and its proper place is on the server. When 
you upload a picture to Flickr, you create a new resource: the new picture has its own 
URI and can be the target of future requests. You can fetch, modify, and delete the 
"picture" resource through HTTP. It's there for everybody: I can fetch it too. The pic- 
ture is a bit of resource state, and it stays on the server until a client deletes it. 

Client state can show up when you don't expect it. Lots of web services make you sign 
up for a unique string they call an API key or application key. You send in this key with 
every request, and the server uses it to restrict you to a certain number of requests a 
day. For instance, an API key for Google's deprecated SOAP search API is good for 
1,000 requests a day. That's client state: it's different for every client. Once you exceed 
the limit, the behavior of the service changes dramatically: on request 1,000 you get 
your data, and on request 1,001 you get an error. Meanwhile, I'm on request 402 and 
the service still works fine for me. 



90 | Chapter 4: The Resource-Oriented Architecture 



Of course, clients can't be trusted to self-report this bit of application state: the temp- 
tation to cheat is too great. So servers keep this kind of application state on the server, 
violating statelessness. The API key is like the Rails _session_id cookie, a key into a 
server-side client session that lasts one day. This is fine as far as it goes, but there's a 
scalability price to be paid. If the service is to be distributed across multiple machines, 
every machine in the cluster needs to know that you're on request 1,001 and I'm on 
request 402 (technical term: session replication), so that every machine knows to deny 
you access and let me through. Alternatively, the load balancer needs to make sure that 
every one of your requests goes to the same machine in the cluster (technical term: 
session affinity). Statelessness removes this requirement. As a service designer, you only 
need to start thinking about data replication when your resource state needs to be split 
across multiple machines. 

Representations 

When you split your application into resources, you increase its surface area. Your users 
can construct an appropriate URI and enter your application right where they need to 
be. But the resources aren't the data; they're just the service designer's idea of how to 
split up the data into "a list of open bugs" or "information about jellyfish. " A web server 
can't send an idea; it has to send a series of bytes, in a specific file format, in a specific 
language. This is a representation of the resource. 

A resource is a source of representations, and a representation is just some data about 
the current state of a resource. Most resources are themselves items of data (like a list 
of bugs), so an obvious representation of a resource is the data itself. The server might 
present a list of open bugs as an XML document, a web page, or as comma-separated 
text. The sales numbers for the last quarter of 2004 might be represented numerically 
or as a graphical chart. Lots of news sites make their articles available in an ad-laden 
format, and in a stripped-down "printer-friendly" format. These are all different rep- 
resentations of the same resources. 

But some resources represent physical objects, or other things that can't be reduced to 
information. What's a good representation for such things? You don't need to worry 
about perfect fidelity: a representation is any useful information about the state of a 
resource. 

Consider a physical object, a soda machine, hooked up to a web service JlThe goal is 
to let the machine's customers avoid unnecessary trips to the machine. With the service, 
customers know when the soda is cold, and when their favorite brand is sold out. 
Nobody expects the physical cans of soda to be made available through the web service, 
because physical objects aren't data. But they do have data about them: metadata. Each 

II This idea is based on the CMU Coke machine (http://www.cs.cmu.edu/%7Ecoke/), which for many years was 
observed by instruments and whose current state was accessible through the Finger protocol. The machine 
is still around, though at the time of writing its state was not accessible online. 



Representations | 91 



slot in the soda machine can be instrumented with a device that knows about the flavor, 
price, and temperature of the next available can of soda. Each slot can be exposed as a 
resource, and so can the soda machine as a whole. The metadata from the instruments 
can be used in representations of the resources. 

Even when one of an object's representations contains the actual data, it may also have 
representations that contain metadata. An online bookstore may serve two represen- 
tations of a book: 

1. One containing only metadata, like a cover image and reviews, used to advertise 
the book. 

2. An electronic copy of the data in the book, sent to you via HTTP when you pay 
for it. 

Representations can flow the other way, too. You can send a representation of a new 
resource to the server and have the server create the resource. This is what happens 
when you upload a picture to Flickr. Or you can give the server a new representation 
of an existing resource, and have the server modify the resource to bring it in line with 
the new representation. 

Deciding Between Representations 

If a server provides multiple representations of a resource, how does it figure out which 
one the client is asking for? For instance, a press release might be put out in both English 
and Spanish. Which one does a given client want? 

There are a number of ways to figure this out within the constraints of REST. The 
simplest, and the one I recommend for the Resource-Oriented Architecture, is to give 
a distinct URI to each representation of a resource, http://www.example.com/releases/ 
104. en could designate the English representation of the press release, and http:// 
www.example.com/releases/104.es could designate the Spanish representation. 

I recommend this technique for ROA applications because it means the URI contains 
all information necessary for the server to fulfill the request. The disadvantage, as 
whenever you expose multiple URIs for the same resource, is dilution: people who talk 
about the press release in different languages appear to be talking about different things. 
You can mitigate this problem somewhat by exposing the URI http://www.exam- 
ple.com/releases/104 to mean the release as a Platonic form, independent of any lan- 
guage. 

The alternative way is called content negotiation. In this scenario the only exposed URI 
is the Platonic form URI, http://www.example.com/releases/104. When a client makes 
a request for that URI, it provides special HTTP request headers that signal what kind 
of representations the client is willing to accept. 

Your web browser has a setting for language preferences: which languages you'd prefer 
to get web pages in. The browser submits this information with every HTTP request, 



92 | Chapter 4: The Resource-Oriented Architecture 



in the Accept- Language header. The server usually ignores this information because 
most web pages are available in only one language. But it fits with what we're trying to 
do here: expose different-language representations of the same resource. When a client 
requests http://www.example.com/releases/104, the server can decide whether to serve 
the English or the Spanish representation based on the client's Accept- Language header. 

The Google search engine is a good place to try this out. You can get 
your search results in almost any language by changing your browser 
f jf; language settings, or by manipulating the hi query variable in the URI 
(for instance, hl=tr for Turkish) . The search engine supports both con- 
tent negotiation and different URIs for different representations. 

A client can also set the Accept header to specify which file format it prefers for repre- 
sentations. A client can say it prefers XHTML to HTML, or SVG to any other graphics 
format. 

The server is allowed to use any of this request metadata when deciding which repre- 
sentation to send. Other types of request metadata include payment information, au- 
thentication credentials, the time of the request, caching directives, and even the IP 
address of the client. All of these might make a difference in the server's decision of 
what data to include in the representation, which language and which format to use, 
and even whether to send a representation at all or to deny access. 

It's RESTful to keep this information in the HTTP headers, and it's RESTful to put it 
in the URI. I recommend keeping as much of this information as possible in the URI, 
and as little as possible in request metadata. I think URIs are more useful than metadata. 
URIs get passed around from person to person, and from program to program. The 
request metadata almost always gets lost in transition. 

Here's a simple example of this dilemma: the W3C HTML validator, a web service 
available at http://validator.w3.org/. Here's a URI to a resource on the W3C's site, a 
validation report on the English version of my hypothetical press release: http://vali 
dator.w3.org/check?uri=http%3A%2F%2Fwww. example. com%2Freleases%2Fl04. en. 

Here's another resource: a validation report on the Spanish version of the press release: 
http : //validator .w3 . org/ check?uri=http%3A%2F%2Fwww. example . com%2Freleases 
%2Fl04.es. 

Every URI in your web space becomes a resource in the W3C's web application, 
whether or not it designates a distinct resource on your site. If your press release has a 
separate URI for each representation, you can get two resources from the W3C: vali- 
dation reports for the English and the Spanish versions of the press release. 

But if you only expose the Platonic form URI, and serve both representations from that 
URI, you can only get one resource from the W3C. That would be a validation report 
for the default version of the press release (probably the English one). You've got no 
way of knowing whether or not the Spanish representation contains HTML formatting 



Representations | 93 



Part of the directory 

| 

lellyfish: A Dangerous Ocean Organism of Hawaii 
A general discussion of jellyfish or sea jellies, especially the box jellyfish, 
Carybdea alata and C, rastonii, which regularly swarm in Hawaiian waters, 
www.aloha.com/~lifeguards/jelyfish.html - 31k - Cached - Similar pages 



Goooooooooog le ► 

ResultPage: 1 2 3 4 5 6 7 B 9 10 Next 

1 

Internal directory links 



Figure 4-3. Closeup on a page of Google search results 

errors. If the server doesn't expose the Spanish press release as its own URI, there's no 
corresponding resource available on the W3C site. This doesn't mean you can't expose 
that Platonic form URI: just that it shouldn't be the only URI you use. 

Unlike humans, computer programs are very bad at dealing with representations they 
didn't expect. I think an automated web client should be as explicit as possible about 
the representation it wants. This almost always means specifying a representation in 
the URL. 



Links and Connectedness 

Sometimes representations are little more than serialized data structures. They're in- 
tended to be sucked of their data and discarded. But in the most RESTful services, 
representations are hypermedia: documents that contain not just data, but links to 
other resources. 

Let's take the search example again. If you go to Google's directory of documents about 
jellyfish (http://www. google. com/search?q=jellyfish), you see some search results, and a 
set of internal links to other pages of the directory. Figure 4-3 shows a representative 
sample of the page. 

There's data here, and links. The data says that somewhere on the Web, someone said 
such-and-such about jellyfish, with emphasis on two species of Hawaiian jellyfish. The 
links give you access to other resources: some within the Google search "web service," 
and some elsewhere on the Web: 

• The external web page that talks about jellyfish: http://www.aloha.com/~life 
guards/ jelyfish.html. The main point of this web service, of course, is to present 
links of this sort. 

• A link to a Google-provided cache of the external page (the "Cached" link) . These 
links always have long URIs that point to Google-owned IP addresses, like http:// 
209 . 85 • 165 • 104/search?q=cache : FQrLzPUOtKQ] . . . 



94 | Chapter 4: The Resource-Oriented Architecture 



• A link to a directory of pages Google thinks are related to the external page (http:// 
www. google. com/search?q=related:www. aloha.com/~lifeguards/jely fish.html, 
linked as "Similar pages"). This is another case of a web service taking a URI as 
input. 

• A set of navigation links that take you to different pages of the "jellyfish" directory: 
http://www. google. com/search?q=jellyfish&start=10, http://www.google.com/ 
search?q=jellyfish&start=20, and so on. 

Earlier in this chapter, I showed what might happen if HTTP was a stateful protocol 
like FTP. Figure 4-2 shows the paths a stateful HTTP client might take during a "ses- 
sion" with www.google.com. HTTP doesn't really work that way, but that figure does a 
good job of showing how we use the human web. To use a search engine we start at 
the home page, fill out a form to do a search, and then click links to go to subsequent 
pages of results. We don't usually type in one URI after another: we follow links and 
fill out forms. 

If you've read about REST before, you might have encountered an axiom from the 
Fielding dissertation: "Hypermedia as the engine of application state." This is what 
that axiom means: the current state of an HTTP "session" is not stored on the server 
as a resource state, but tracked by the client as an application state, and created by the 
path the client takes through the Web. The server guides the client's path by serving 
"hypermedia": links and forms inside hypertext representations. 

The server sends the client guidelines about which states are near the current one. The 
"next" link on http:/ /www. google.com/ search?q=)ellyfish is a lever of state: it shows you 
how to get from the current state to a related one. This is very powerful. A document 
that contains a URI points to another possible state of the application: "page two," or 
"related to this URI," or "a cached version of this URI." Or it may be pointing to a 
possible state of a totally different application. 

I'm calling the quality of having links connectedness. A web service is connected to the 
extent that you can put the service in different states just by following links and filling 
out forms. I'm calling this "connectedness" because "hypermedia as the engine of ap- 
plication state" makes the concept sound more difficult than it is. All I'm saying is that 
resources should link to each other in their representations. 

The human web is easy to use because it's well connected. Any experienced user knows 
how to type URIs into the browser's address bar, and how to jump around a site by 
modifying the URI, but many users do all their web surfing from a single starting point: 
the browser home page set by their ISP. This is possible because the Web is well con- 
nected. Pages link to each other, even across sites. 

But most web services are not internally connected, let alone connected to each other. 
Amazon S3 is a RESTful web service that's addressible and stateless, but not connected. 
S3 representations never include URIs. To GET an S3 bucket, you have to know the 
rules for constructing the bucket's URI. You can't just GET the bucket list and follow 
a link to the bucket you want. 



Links and Connectedness | 95 



— ► 



a b c 

All three services expose the same functionality, but their usability increases toward the right. 

•Service A is a typical RPC-style service, exposing everything through a single URI. It's neither addressable 
nor connected. 

•Service B is addressable but not connected: there are no indications of the relationships 
between resources.This might be a REST-RPC hybrid service, or a RESTful service like Amazon S3. 

•Service C is addressable and well-connected: resources are linked to each other in ways that (presumably) 
make sense. This could be a fully RESTful service. 



Figure 4-4. One service three ways 

Example 4-1 shows an S3 bucket list that I've changed (I added a URI tag) so that it's 
connected. Compare to Example 3-5, which has no URI tag. This is just one way of 
introducing URIs into an XML representation. As resources become better-connected, 
the relationships between them becomes more obvious (see Figure 4-4). 

Example 4-1. A connected "list of your buckets" 
<?xml version='l.O' encoding='UTF-8' ?> 

<ListAllMyBucketsResult xmlns=' http://s3.amazonaws.com/doc/2006-03-Ol/'> 
<0wner> 

<ID>c0363f7260f2f5fcf38d48039f4fb5cab21b0605778l7310be5170e7774aad70</ID> 

<DisplayName>leonardr28</DisplayName> 
</Owner> 
<Buckets> 

<Bucket> 
<Name>crummy.com</Name> 

<URI>https : //s3 . amazonaws . com/crummy . com</URI> 
<CreationDate>2006-10-26Tl8:46:45.000Z</CreationDate> 
</Bucket> 
</Buckets> 
</ListAllMyBucketsResult> 



The Uniform Interface 

All across the Web, there are only a few basic things you can do to a resource. HTTP 
provides four basic methods for the four most common operations: 

• Retrieve a representation of a resource: HTTP GET 



96 I Chapter 4: The Resource-Oriented Architecture 



• Create a new resource: HTTP PUT to a new URI, or HTTP POST to an existing 
URI (see the "POST" section below) 

• Modify an existing resource: HTTP PUT to an existing URI 

• Delete an existing resource: HTTP DELETE 

I'll explain how these four are used to represent just about any operation you can think 
of. I'll also cover two HTTP methods for two less common operations: HEAD and 
OPTIONS. 

GET, PUT, and DELETE 

These three should be familiar to you from the S3 example in Chapter 3. To fetch or 
delete a resource, the client just sends a GET or DELETE request to its URI. In the case 
of a GET request, the server sends back a representation in the response entity-body. 
For a DELETE request, the response entity-body may contain a status message, or 
nothing at all. 

To create or modify a resource, the client sends a PUT request that usually includes an 
entity-body. The entity-body contains the client's proposed new representation of the 
resource. What data this is, and what format it's in, depends on the service. Whatever 
it looks like, this is the point at which application state moves onto the server and 
becomes resource state. 

Again, think of the S3 service, where there are two kinds of resources you can create: 
buckets and objects. To create an object, you send a PUT request to its URI and include 
the object's content in the entity-body of your request. You do the same thing to modify 
an object: the new content overwrites any old content. 

Creating a bucket is a little different because you don't have to specify an entity-body 
in the PUT request. A bucket has no resource state except for its name, and the name 
is part of the URI. (This is not quite true. The objects in a bucket are also elements of 
that bucket's resource state: after all, they're listed when you GET a bucket's repre- 
sentation. But every S3 object is a resource of its own, so there's no need to manipu- 
late an object through its bucket. Every object exposes the uniform interface and you 
can manipulate it separately.) Specify the bucket's URI and you've specified its repre- 
sentation. PUT requests for most resources do include an entity-body containing a 
representation, but as you can see it's not a requirement. 

HEAD and OPTIONS 

There are three other HTTP methods I consider part of the uniform interface. Two of 
them are simple utility methods, so I'll cover them first. 

• Retrieve a metadata-only representation: HTTP HEAD 

• Check which HTTP methods a particular resource supports: HTTP OPTIONS 



The Uniform Interface | 97 



You saw the HEAD method exposed by the S3 services's resources in Chapter 3. An 
S3 client uses HEAD to fetch metadata about a resource without downloading the 
possibly enormous entity-body. That's what HEAD is for. A client can use HEAD to 
check whether a resource exists, or find out other information about the resource, 
without fetching its entire representation. HEAD gives you exactly what a GET request 
would give you, but without the entity-body. 



The OPTIONS method lets the client discover what it's allowed to do to a resource. 
The response to an OPTIONS request contains the HTTP Allow header, which lays out 
the subset of the uniform interface this resource supports. Here's a sample Allow header: 
Allow: GET, HEAD 

That particular header means the client can expect the server to act reasonably to a 
GET or HEAD request for this resource, but that the resource doesn't support any other 
HTTP methods. Effectively, this resource is read-only. 

The headers the client sends in the request may affect the Allow header the server sends 
in response. For instance, if you send a proper Authorization header along with an 
OPTIONS request, you may find that you're allowed to make GET, HEAD, PUT, and 
DELETE requests against a particular URI. If you send the same OPTIONS request but 
omit the Authorization header, you may find that you're only allowed to make GET 
and HEAD requests. The OPTIONS method lets the client do simple access control 
checks. 

In theory, the server can send additional information in response to an OPTIONS re- 
quest, and the client can send OPTIONS requests that ask very specific questions about 
the server's capabilities. Very nice, except there are no accepted standards for what a 
client might ask in an OPTIONS request. Apart from the Allow header there are no 
accepted standards for what a server might send in response. Most web servers and 
frameworks feature very poor support for OPTIONS. So far, OPTIONS is a promising 
idea that nobody uses. 



Now we come to that most misunderstood of HTTP methods: POST. This method 
essentially has two purposes: one that fits in with the constraints of REST, and one that 
goes outside REST and introduces an element of the RPC style. In complex cases like 
this it's best to go back to the original text. Here's what RFC 2616, the HTTP standard, 
says about POST (this is from section 9.5): 

POST is designed to allow a uniform method to cover the following functions: 




There are two standard HTTP methods I don't cover in this book: 
TRACE and CONNECT. TRACE is used to debug proxies, and CON- 
fl-T; NECT is used to forward some other protocol through an HTTP proxy. 



POST 



98 | Chapter 4: The Resource-Oriented Architecture 



• Annotation of existing resources; 

• Posting a message to a bulletin board, newsgroup, mailing list, or similar group of 
articles; 

• Providing a block of data, such as the result of submitting a form, to a data-handling 
process; 

• Extending a database through an append operation. 

The actual function performed by the POST method is determined by the server and is 
usually dependent on the Request-URI. The posted entity is subordinate to that URI in 
the same way that a file is subordinate to a directory containing it, a news article is 
subordinate to a newsgroup to which it is posted, or a record is subordinate to a database. 

What does this mean in the context of REST and the ROA? 
Creating subordinate resources 

In a RESTful design, POST is commonly used to create subordinate resources: resources 
that exist in relation to some other "parent" resource. A weblog program may expose 
each weblog as a resource (/weblogs/myweblog), and the individual weblog entries as 
subordinate resources (/weblogs/myweblog/entries/l). A web-enabled database may 
expose a table as a resource, and the individual database rows as its subordinate re- 
sources. To create a weblog entry or a database record, you POST to the parent: the 
weblog or the database table. What data you post, and what format it's in, depends on 
the service, but as with PUT, this is the point where application state becomes resource 
state. You may see this use of POST called POST(a), for "append". When I say "POST" 
in this book, I almost always mean POST (a). 

Why can't you just use PUT to create subordinate resources? Well, sometimes you can. 
An S3 object is a subordinate resource: every S3 object is contained in some S3 bucket. 
But we don't create an S3 object by sending a POST request to the bucket. We send a 
PUT request directly to the URI of the object. The difference between PUT and POST 
is this: the client uses PUT when it's in charge of deciding which URI the new resource 
should have. The client uses POST when the server is in charge of deciding which URI 
the new resource should have. 

The S3 service expects clients to create S3 objects with PUT, because an S3 object's 
URI is completely determined by its name and the name of the bucket. If the client 
knows enough to create the object, it knows what its URI will be. The obvious URI to 
use as the target of the PUT request is the one the bucket will live at once it exists. 

But consider an application in which the server has more control over the URIs: say, a 
weblog program. The client can gather all the information necessary to create a weblog 
entry, and still not know what URI the entry will have once created. Maybe the server 
bases the URIs on ordering or an internal database ID: will the final URI be /weblogs/ 
myweblog/entries/l or /weblogs/myweblog/entries/1000? Maybe the final URI is based 
on the posting time: what time does the server think it is? The client shouldn't have to 
know these things. 



The Uniform Interface | 99 



The POST method is a way of creating a new resource without the client having to 
know its exact URI. In most cases the client only needs to know the URI of a "parent" 
or "factory" resource. The server takes the representation from the entity-body and use 
it to create a new resource "underneath" the "parent" resource (the meaning of "un- 
derneath" depends on context). 

The response to this sort of POST request usually has an HTTP status code of 201 
("Created"). Its Location header contains the URI of the newly created subordinate 
resource. Now that the resource actually exists and the client knows its URI, future 
requests can use the PUT method to modify that resource, GET to fetch a representation 
of it, and DELETE to delete it. 

Table 4-1 shows how a PUT request to a URI might create or modify the underlying 
resource; and how a POST request to the same URI might create a new, subordinate 
resource. 

Table 4-1. PUT actions 





PUT to a new re- 


PUT to an existing 
resource 






source 




/weblogs 


N/A (resource al- 
ready exists) 


No effect 


Create a new weblog 


/weblogs/myweblog 


Create this weblog 


Modify this Weblog's 
settings 


Create a new weblog entry 


/weblogs/myweblog/ 
entries/l 


N/A (how would you 
get this URI?) 


Edit this weblog en- 
try 


Post a comment to this weblog entry 



Appending to the resource state 

The information conveyed in a POST to a resource doesn't have to result in a whole 
new subordinate resource. Sometimes when you POST data to a resource, it appends 
the information you POSTed to its own state, instead of putting it in a new resource. 

Consider an event logging service that exposes a single resource: the log. Say its URI 
is /log. To get the log you send a GET request to /log. 

Now, how should a client append to the log? The client might send a PUT request 
to /log, but the PUT method has the implication of creating a new resource, or 
overwriting old settings with new ones. The client isn't doing either: it's just appending 
information to the end of the log. 

The POST method works here, just as it would if each log entry was exposed as a 
separate resource. The semantics of POST are the same in both cases: the client adds 
subordinate information to an existing resource. The only difference is that in the case 
of the weblog and weblog entries, the subordinate information showed up as a new 
resource. Here, the subordinate information shows up as new data in the parent's rep- 
resentation. 



100 I Chapter 4: The Resource-Oriented Architecture 



Overloaded POST: The not-so-uniform interface 

That way of looking at things explains most of what the HTTP standard says about 
POST. You can use it to create resources underneath a parent resource, and you can 
use it to append extra data onto the current state of a resource. The one use of POST 
I haven't explained is the one you're probably most familiar with, because it's the one 
that drives almost all web applications: providing a block of data, such as the result of 
submitting a form, to a data-handling process. 

What's a "data-handling process"? That sounds pretty vague. And, indeed, just about 
anything can be a data-handling process. Using POST this way turns a resource into a 
tiny message processor that acts like an XML-RPC server. The resource accepts a POST 
request, examines the request, and decides to do... something. Then it decides to serve 
to the client... some data. 

I call this use of POST overloaded POST, by analogy to operator overloading in a pro- 
gramming language. It's overloaded because a single HTTP method is being used to 
signify any number of non-HTTP methods. It's confusing for the same reason operator 
overloading can be confusing: you thought you knew what HTTP POST did, but now 
it's being used to achieve some unknown purpose. You might see overloaded POST 
called POST(p), for "process." 

When your service exposes overloaded POST, you reopen the question: "why should 
the server do this instead of that?" Every HTTP request has to contain method infor- 
mation, and when you use overloaded POST it can't go into the HTTP method. The 
POST method is just a directive to the server, saying: "Look inside the HTTP request 
for the real method information." The real information may be in the URI, the HTTP 
headers, or the entity-body. However it happens, an element of the RPC style has crept 
into the service. 

When the method information isn't found in the HTTP method, the interface stops 
being uniform. The real method information might be anything. As a REST partisan I 
don't like this very much, but occasionally it's unavoidable. By Chapter 9 you'll have 
seen how just about any scenario you can think of can be exposed through HTTP's 
uniform interface, but sometimes the RPC style is the easiest way to express complex 
operations that span multiple resources. 

You may need to expose overloaded POST even if you're only using POST to create 
subordinate resources or to append to a resource's representation. What if a single 
resource supports both kinds of POST? How does the server know whether a client is 
POSTing to create a subordinate resource, or to append to the existing resource's rep- 
resentation? You may need to put some additional method information elsewhere in 
the HTTP request. 

Overloaded POST should not be used to cover up poor resource design. Remember, a 
resource can be anything. It's usually possible to shuffle your resource design so that 
the uniform interface applies, rather than introduce the RPC style into your service. 



The Uniform Interface | 101 



Safety and Idempotence 

If you expose HTTP's uniform interface as it was designed, you get two useful properties 
for free. When correctly used, GET and HEAD requests are safe. GET, HEAD, PUT 
and DELETE requests are idempotent. 

Safety 

A GET or HEAD request is a request to read some data, not a request to change any 
server state. The client can make a GET or HEAD request 10 times and it's the same 
as making it once, or never making it at all. When you GET http://www.google.com/ 
search?q=jellyfish, you aren't changing anything about the directory of jellyfish resour- 
ces. You're just retrieving a representation of it. A client should be able to send a GET 
or HEAD request to an unknown URI and feel safe that nothing disastrous will happen. 

This is not to say that GET and HEAD requests can't have side effects. Some resources 
are hit counters that increment every time a client GETs them. Most web servers log 
every incoming request to a log file. These are side effects: the server state, and even 
the resource state, is changing in response to a GET request. But the client didn't ask 
for the side effects, and it's not responsible for them. A client should never make a GET 
or HEAD request just for the side effects, and the side effects should never be so big 
that the client might wish it hadn't made the request. 

Idempotence 

Idempotence is a slightly tricker notion. The idea comes from math, and if you're not 
familiar with idempotence, a math example might help. An idempotent operation in 
math is one that has the same effect whether you apply it once, or more than once. 
Multiplying a number by zero is idempotent: 4 x0 x0 x0 is the same as 4 xO.#By analogy, 
an operation on a resource is idempotent if making one request is the same as making 
a series of identical requests. The second and subsequent requests leave the resource 
state in exactly the same state as the first request did. 

PUT and DELETE operations are idempotent. If you DELETE a resource, it's gone. If 
you DELETE it again, it's still gone. If you create a new resource with PUT, and then 
resend the PUT request, the resource is still there and it has the same properties you 
gave it when you created it. If you use PUT to change the state of a resource, you can 
resend the PUT request and the resource state won't change again. 

The practical upshot of this is that you shouldn't allow your clients to PUT represen- 
tations that change a resource's state in relative terms. If a resource keeps a numeric 
value as part of its resource state, a client might use PUT to set that value to 4, or 0, or 
-50, but not to increment that value by 1. If the initial value is 0, sending two PUT 

* Multiplying a number by one is both safe and idempotent: 4xlxlxl is the same as 4 x 1 , which is the same 
as 4. Multiplication by zero is not safe, because 4 x0 is not the same as 4. Multiplication by any other number 
is neither safe nor idempotent. 



102 | Chapter 4: The Resource-Oriented Architecture 



requests that say "set the value to 4" leaves the value at 4. If the initial value is 0, sending 
two PUT requests that say "increment the value by 1" leaves the value not at 1, but at 
2. That's not idempotent. 

Why safety and idempotence matter 

Safety and idempotence let a client make reliable HTTP requests over an unreliable 
network. If you make a GET request and never get a response, just make another one. 
It's safe: even if your earlier request went through, it didn't have any real effect on the 
server. If you make a PUT request and never get a response, just make another one. If 
your earlier request got through, your second request will have no additional effect. 

POST is neither safe nor idempotent. Making two identical POST requests to a "fac- 
tory" resource will probably result in two subordinate resources containing the same 
information. With overloaded POST, all bets are off. 

The most common misuse of the uniform interface is to expose unsafe operations 
through GET. The del.icio.us and Flickr APIs both do this. When you GET https:// 
api.del.icio.us/posts/delete, you're not fetching a representation: you're modifying the 
del.icio.us data set. 

Why is this bad? Well, here's a story. In 2005 Google released a client-side caching tool 
called Web Accelerator. It runs in conjunction with your web browser and "pre-fetch- 
es" the pages linked to from whatever page you're viewing. If you happen to click one 
of those links, the page on the other side will load faster, because your computer has 
already fetched it. 

Web Accelerator was a disaster. Not because of any problem in the software itself, but 
because the Web is full of applications that misuse GET. Web Accelerator assumed 
that GET operations were safe, that clients could make them ahead of time just in case 
a human being wanted to see the corresponding representations. But when it made 
those GET requests to real URIs, it changed the data sets. People lost data. 

There's plenty of blame to go around: programmers shouldn't expose unsafe actions 
through GET, and Google shouldn't have released a real-world tool that didn't work 
with the real -wo rid web. The current version of Web Accelerator ignores all URIs that 
contain query variables. This solves part of the problem, but it also prevents many 
resources that are safe to use through GET (such as Google web searches) from being 
pre-f etched. 

Multiply the examples if you like. Many web services and web applications use URIs 
as input, and the first thing they do is send a GET request to fetch a representation of 
a resource. These services don't mean to trigger catastrophic side effects, but it's not 
up to them. It's up to the service to handle a GET request in a way that complies with 
the HTTP standard. 



The Uniform Interface | 103 



Why the Uniform Interface Matters 

The important thing about REST is not that you use the specific uniform interface that 
HTTP defines. REST specifies a uniform interface, but it doesn't say which uniform 
interface. GET, PUT, and the rest are not a perfect interface for all time. What's im- 
portant is the uniformity: that every service use HTTP's interface the same way. 

The point is not that GET is the best name for a read operation, but that GET means 
"read" across the Web, no matter which resource you're using it on. Given a URI of a 
resource, there's no question of how you get a representation: you send an HTTP GET 
request to that URI. The uniform interface makes any two services as similar as any 
two web sites. Without the uniform interface, you'd have to learn how each service 
expected to receive and send information. The rules might even be different for different 
resources within a single service. 

You can program a computer to understand what GET means, and that understanding 
will apply to every RESTful web service. There's not much to understand. The service- 
specific code can live in the handling of the representation. Without the uniform in- 
terface, you get a multiplicity of methods taking the place of GET: doSearch and get 
Page and nextPrime. Every service speaks a different language. This is also the reason I 
don't like overloaded POST very much: it turns the simple Esperanto of the uniform 
interface into a Babel of one-off sublanguages. 

Some applications extend HTTP's uniform interface. The most obvious case is Web- 
DAV, which adds eight new HTTP methods including MOVE, COPY, and SEARCH. 
Using these methods in a web service would not violate any precept of REST, because 
REST doesn't say what the uniform interface should look like. Using them would vio- 
late my Resource-Oriented Architecture (I've explicitly tied the ROA to the standard 
HTTP methods), but your service could still be resource-oriented in a general sense. 

The real reason not to use the WebDAV methods is that doing so makes your service 
incompatible with other RESTful services. Your service would use a different uniform 
interface than most other services. There are web services like Subversion that use the 
WebDAV methods, so your service wouldn't be all alone. But it would be part of a 
much smaller web. This is why making up your own HTTP methods is a very, very bad 
idea: your custom vocabulary puts you in a community of one. You might as well be 
using XML-RPC. 

Another uniform interface consists solely of HTTP GET and overloaded POST. To 
fetch a representation of a resource, you send GET to its URI. To create, modify, or 
delete a resource, you send POST. This interface is perfectly RESTful, but, again, it 
doesn't conform to my Resource-Oriented Architecture. This interface is just rich 
enough to distinguish between safe and unsafe operations. A resource-oriented web 
application would use this interface, because today's HTML forms only support GET 
and POST. 



104 | Chapter 4: The Resource-Oriented Architecture 



That's It! 



That's the Resource-Oriented Architecture. It's just four concepts: 

1. Resources 

2. Their names (URIs) 

3. Their representations 

4. The links between them 

and four properties: 

1. Addressability 

2. Statelessness 

3. Connectedness 

4. A uniform interface 

Of course, there are still a lot of open questions. How should a real data set be split 
into resources, and how should the resources be laid out? What should go into the 
actual HTTP requests and responses? I'm going to spend much of the rest of the book 
exploring issues like these. 



That's It! | 105 



CHAPTER 5 

Designing Read-Only Resource- 
Oriented Services 



We've got some information we want to expose to people elsewhere on the network. 
We want to reach the widest possible combination of clients. Every programming lan- 
guage has an HTTP library, so the natural choice is to expose the data over HTTP. 
Every programming language has an XML parsing library, so we can format the data 
with XML and always be understood. Wheel 

Sometimes that's as far as the train of thought goes. The solution is obvious, so the 
programmers set to work. Despite its vagueness, this technique gives surprisingly good 
results. Most people are intuitively familiar with what makes a good web site, and a 
good web service works much the same way. 

Unfortunately, this gut-feeling approach combines everyone's gut feelings into a stew 
of web services that are usually not RESTful (they're REST-RPC hybrids), and which 
work alike only in superficial ways. If you understand why REST works, you can make 
your services safer, easier to use, and accessible through standard tools. 

Some "web services" were never intended to be used as such, and have RESTful qualities 
seemingly by accident. Into this category fall the many well-designed web sites that 
have been screen-scraped over the years. So do many providers of images: for instance, 
the static map tiles served up to the Google Maps application, where you change the 
URI to address a different part of the Earth. An amusing example is Amazon product 
images, which can be manipulated in funny ways by putting extra strings in the URL* 

It is no accident that so many web sites are RESTful. A well-designed web site presents 
uncluttered representations of sensibly named resources, accessible through HTTP 
GET. Uncluttered representations are easy to parse or screen-scrape, and sensibly 
named resources are easy to address programmatically. Using GET to fetch a 

This trick is detailed in Nat Gertler's enjoyable article, "Abusing Amazon Images" (http://ivww.aaugh.com/ 
imageabuse.html) . 



107 



representation respects HTTP's uniform interface. Design a web site by these rules, and 
it will fit well with my Resource-Oriented Architecture. 

Now that I've introduced the principles of REST, within the ROA, I'll show how to use 
the ROA to design programmatic services that serve data across the network. These 
simple services provide client access to a data set. They may even let clients filter or 
search the data. But they don't let clients modify the data or add to it. In Chapter 6 I 
talk about web services that let you store and modify information on the server. For 
now I'm focused on letting clients retrieve and search a data set. 

I've split the discussion because many excellent web services do nothing more than 
send useful information out to the people that need it. These are not toy services. Any 
web-based database search falls into this category: web searches, book searches, even 
the stereotypical stock quote web service (okay, that one's probably just a toy). It's 
more manageable to cover the simpler cases — which do happen in real life — than to 
try to cover everything in one huge chapter. The lessons in the next chapter build 
directly on what I say in this one. After all, a web service that lets clients modify infor- 
mation must also let them retrieve it. 

In this chapter I design a web service that serves information about maps. It's inspired 
by web applications like Google Maps, but those sites (and the third-party sites build 
atop them) are designed for ad hoc use by humans. As with any well-designed web site, 
you can consume Google Maps image tiles as a web service, but only somewhat illicitly 
and with difficulty. The fantasy service I design here is a programmer-friendly way to 
retrieve map data for any purpose, including a browser-based map navigation applica- 
tion like the Google Maps Ajax application. 

I won't actually implement this service. An implementation would be too complex to 
fit in this book, and I don't own the necessary data anyway. (Note, though, that in 
Chapter 7 I use the lessons of this chapter to implement a social bookmarking service 
similar to del.icio.us). This chapter and the next aim to teach you how to see a problem 
from a resource-oriented point of view. Along the way I hope to demonstrate that the 
ROA's simple rules and uniform interface can represent an extremely powerful and 
fairly complex distributed service. 

Resource Design 

The standard design technique for object-oriented programs is to break a system down 
into its moving parts: its nouns. An object is something. Each noun ("Reader," "Col- 
umn," "Story," "Comment") gets its own class, and behavior for interacting with the 
other nouns. By contrast, a good design technique for an RPC-style architecture is to 
break the system into its motions: its verbs. A procedure does something ("Subscribe 
to," "Read," "Comment on"). 

A resource is something, so I take an object-oriented approach to designing resources. 
In fact, the resource-oriented design strategy could be called "extreme object-oriented. " 



108 | Chapter 5: Designing Read-Only Resource-Oriented Services 



A class in a programming language can expose any number of methods and give them 
any names, but an HTTP resource exposes a uniform interface of at most six HTTP 
methods. These methods allow only the most basic operations: create (PUT or POST), 
modify (PUT), read (GET), and delete (DELETE). If necessary, you can extend this 
interface by overloading POST, turning a resource into a small RPC-style message pro- 
cessor, but you shouldn't need to do that very often. 

A service can expose a Story resource, and a Story can exist in either draft or published 
form, but a client can't publish a draft Story to the live site. Not in so many words, 
anyway: "publish" isn't one of the six actions. A client can PUT a new representation 
for the Story which depicts it as published. The resource may then be available at a new 
URI, and may no longer require authentication to read. This is a subtle distinction, but 
one that keeps you from making dangerous design mistakes like exposing a special 
RPC-style "publish this article" URI through GET. 

The uniform interface means that a resource-oriented design must treat as objects what 
an object-oriented design might consider verbs. In the ROA, a Reader can't subscribe 
to a regularly appearing Column, because "subscribe to" is not part of the uniform 
interface. There must be a third object, Subscription, representing that relationship 
between a Reader and a Column. This relationship object is subject to the uniform 
interface: it can be created, fetched (perhaps as a syndication feed), and deleted. "Sub- 
scription" might not show up as a first-class object in an object-oriented analysis, but 
it probably would appear as a table in an underlying database model. In a resource- 
oriented analysis, all object manipulation happens through resources that respect the 
uniform interface. Whenever I'm tempted to add a new method to one of my resource 
"classes," I'll resolve the problem by defining a new kind of resource. 

Turning Requirements Into Read-Only Resources 

I've come up with a procedure to follow once you have an idea of what you want your 
program to do.+It produces a set of resources that respond to a read-only subset of 
HTTP's uniform interface: GET and possibly HEAD. Once you get to the end of this 
procedure, you should be ready to implement your resources in whatever language and 
framework you like. If you want to expose a larger subset of the uniform interface, I 
present a slightly extended procedure in Chapter 6. 

1. Figure out the data set 

2. Split the data set into resources 
For each kind of resource: 

3. Name the resources with URIs 

4. Expose a subset of the uniform interface 

t This procedure has a lot in common with Joe Gregorio's "How to create a REST Protocol" (http:// 
www.xml.com/publa/2004/12/01/restful-web.html). 



Turning Requirements Into Read-Only Resources | 109 



5. Design the representation (s) accepted from the client 

6. Design the representation (s) served to the client 

7. Integrate this resource into existing resources, using hypermedia links and forms 

8. Consider the typical course of events: what's supposed to happen? 

9. Consider error conditions: what might go wrong? 

In the sections to come, I'll show, step by step, how following this procedure results in 
a RESTful web service that works like the Web. The only difference between what I do 
and what this procedure says is that I'm going to design all my resources at once, rather 
than take you through the same steps over and over again for each kind of resource. 

Figure Out the Data Set 

A web service starts with a data set, or at least an idea for one. This is the data set you'll 
be exposing and/or getting your users to build. Earlier I said my data set would be maps, 
but which maps? This is a fantasy, so I'll spread the net wide. My imaginary web service 
will serve maps in all projections and at all scales. Maps of the past, the present, and 
the supposed future. Maps of other planets and of individual cities. Political maps, road 
maps (which are just very detailed political maps) , physical maps, geological maps, and 
topographic maps. 

This is not every kind of map. I'll only serve maps that use a standard 2-D coordinate 
system: a way of identifying any given point on the map. The map need not be accurate, 
but it must be addressable (there's that word again) using latitude and longitude. This 
means I won't serve most maps of fictional places, maps that arbitrarily distort geog- 
raphy (the way subway maps do), or maps created before longitude could be measured 
accurately. 

Maps are made out of points: in this case, points of latitude and longitude. Every map 
contains an infinite number of points, but I can have a map without keeping every one 
of those points in my data set. I just need some image data and a couple basic pieces 
of information about the map: what are the latitude and longitude of the map's corners? 
Or, if the map covers an entire planet, where on the map is the prime meridia^^Given 
that information, I can use standard geographical algorithms to locate and move be- 
tween the infinitely many points on a map.§ 

A map is a map of some planet. (I say "planet" for simplicity's sake, but my system will 
serve maps of moons, asteroids, and any other body that has latitude and longitude.) 
A map is an interesting part of my data set, but so is the actual planet it represents. It's 

t Fun fact: prime meridians for planetary bodies are usually chosen by reference to some arbitrary feature like 
a crater. For bodies like Jupiter and Io, whose features are always changing, the prime meridian is defined 
according to which way the body was facing at an arbitrary time. 

§ A good reference for these algorithms is Ed Williams's "Aviation Formulary" (http://williams.best.vwh.net/ 
avform.htm). 



110 | Chapter5: Designing Read-Only Resource-Oriented Services 



convenient to refer to points on a planet, independent of any particular map, even 
though a planet doesn't have physical lines of latitude and longitude running around 
it. One obvious use: I want to be able to see what maps there are for a particular point 
on Earth. There are probably more maps covering a point in New York City than a 
point in the middle of the Pacific Ocean. 

So my data set includes not only the maps and the points on the maps, but the very 
planets themselves, and every point on the planets. It may seem hubristic to treat the 
entire planet Earth as a resource, but remember that I'm not obliged to give a complete 
account of the state of any resource. If my representation of "Earth" is just a list of my 
maps of Earth, that's fine. The important thing is that the client can say "tell me about 
Earth," as opposed to "tell me about the political map of Earth," and I can give an 
answer. 

Speaking of New York City and the Pacific Ocean, some points on a planet are more 
interesting than others. Most points have nothing much underneath them. Some points 
correspond to a cornfield or flat lunar plain, and others correspond to a city or a meteor 
crater. Some points on a planet are places. My users will be disproportionately inter- 
ested in these points on the planets, and the corresponding points on my maps. They 
won't want to specify these places as latitude-longitude pairs. Indeed, many of my users 
will be trying to figure out where something is: they'll be trying to turn a known place 
into a point on a planet. 

Fortunately, most places have agreed-upon names, like "San Francisco," "Eratos- 
thenes," and "Mount Whitney." To make it easy for my users to identify places, my 
data set will include a mapping of place names to the corresponding points on the 
planets." Note that a single planet may have multiple places with the same name. There 
might be one "Joe's Diner" on the Moon and a hundred on Earth, all distinct. If my 
user wants to find a particular Joe's Diner on Earth, they'll have to specify its location 
more precisely than just "Earth." 

What about places that aren't points, like cities, countries, and rivers? For simplicity's 
sake, I'll make a well-chosen point stand for an area on a planet. For instance, I'll have 
a point on Earth near the geographic center of the U.S. that stands for the place called 
"the United States of America." (This is obviously a vast oversimplification. Many real 
GIS mapping programs represent such areas as lists of points, which form lines or 
polygons.) 

Every place is of a certain type. Some places are cities, some mountains, some hot 
springs, some the current locations of ships, some areas of high pollution, and so on. 
I'll keep track of the type of each place. Two places of different types may correspond 



II You may have a private name for a seemingly boring point on the map, like "the cornfield where I kissed 
Betty." This will come into play in Chapter 6 when I expand my web service so that clients can create their 
own place names. For now, I've got a preset database of names for each planet. 



Figure Out the Data Set | 111 



to the same point on a planet: some unfortunate's house may be built on top of a toxic 
waste dump. 

My service can find a place on a planet, given its name, type, or description. It can show 
the place on any appropriate maps, and it can find places nearby. Given a street address, 
my service can locate the corresponding point on the planet Earth, and show it on a 
road map. Given the name of a country, it can locate the corresponding place on the 
planet (as a representative point), and show it on a political map. 

If the client tries to find a place whose name is ambiguous (for instance, "Springfield") 
my service can list all appropriate points within the given scope. The client will also be 
able to search for places of a certain type, without requiring the user give specific names. 
So a user can search for "pollution sites near Reno, Nevada." 

General Lessons 

This is a standard first step in any analysis. Sometimes you get to choose your data set, 
and sometimes you're trying to expose data you've already got. You may come back to 
this step as you see how best to expose your data set as resources. I went through the 
design process two or three times before I figured out that points on a planet needed 
to be considered distinct from points on any particular map. Even now, the data set is 
chaotic, just a bundle of ideas. I'll give it shape when I divide it into resources. 

I presented the results of a search operation ("places on Earth called Springfield") as 
part of the data set. An RPC-oriented analysis would treat these as actions that the client 
invokes — remember doGoogleSearch from the Google SOAP service. Compare this to 
how the Google web site works: in a resource-oriented analysis, ways of looking at the 
data are themselves pieces of data. If you consider an algorithm's output to be a re- 
source, running the algorithm can be as simple as sending a GET to that resource. 

So far I've said nothing about how a web service client can access this data set through 
HTTP. Right now I'm just gathering everything together in one place. I'm also ignoring 
any consideration of how these features should be implemented. If I actually planned 
to provide this service, the features I've announced so far would have a profound effect 
on the structure of my database, and I could start designing that part of the application 
as well. As it is, I'm going to wave away details of the backend implementation, and 
press on with the design of the web service. 

Split the Data Set into Resources 

Once you have a data set in mind, the next step is to decide how to expose the data 
as HTTP resources. Remember that a resource is anything interesting enough to be the 
target of a hypertext link. Anything that might be refered to by name ought to have a 
name. Web services commonly expose three kinds of resources: 



112 | Chapter5: Designing Read-Only Resource-Oriented Services 



Predefined one-off resources for especially important aspects of the application. 

This includes top-level directories of other available resources. Most services ex- 
pose few or no one-off resources. 

Example: A web site's homepage. It's a one-of-a-kind resource, at a well-known 
URI, which acts as a portal to other resources. 

The root URI of Amazon's S3 service (https://s3.amazonaws.com/) serves a list of 
your S3 buckets. There's only one resource of this type on S3. You can GET this 
resource, but you can't DELETE it, and you can't modify it directly: it's modified 
only by operating on its buckets. It's a predefined resource that acts as a directory 
of child resources (the buckets). 

A resource for every object exposed through the service. 

One service may expose many kinds of objects, each with its own resource set. 
Most services expose a large or infinite number of these resources. 

Example: Every S3 bucket you create is exposed as a resource. You can create up 
to 100 buckets, and they can have just about any names you want (it's just that 
your names can't conflict with anyone else's). You can GET and DELETE these 
resources, but once you've created them you can't modify them directly: they're 
modified only by operating on the objects they contain. 

Every S3 object you create is also exposed as a resource. A bucket has room for any 
number of objects. You can GET, PUT, and DELETE these resources as you see fit. 

Resources representing the results of algorithms applied to the data set. 

This includes collection resources, which are usually the results of queries. Most 
services either expose an infinite number of algorithmic resources, or they don't 
expose any. 

Example: A search engine exposes an infinite number of algorithmic resources. 
There's one for every search request you might possibly make. The Google search 
engine exposes one resource at http:// google.com/ search?q=jelly fish (that'd be "a 
directory of resources about jellyfish") and another at http://google.com/search? 
q=chocolate ("a directory of resources about chocolate"). Neither of these resour- 
ces were explicitly defined ahead of time: Google translates any URI of the form 
http:// google. com/search?q={query] into an algorithmic resource "a directory of re- 
sources about {query}." 

I didn't cover this in much detail back in Chapter 3, but S3 also exposes an infinite 
number of algorithmic resources. If you're interested, look back to Example 3-7 
and the implementation of S3: :Bucket#getObjects. Some of S3's algorithmic re- 
sources work like a search engine for the objects in a bucket. If you're only inter- 
ested in objects whose names start with the string "movies/", there's a resource for 
that: it's exposed through the URI https://s3.amazonaws.com/MyBucketPPre 
fix=moviesl. You can GET this resource, but you can't manipulate it directly: it's 
just a view of the underlying data set. 



Split the Data Set into Resources | 113 



Let's apply these categories to my fantasy map service. I need one special resource that 
lists the planets, just as S3 has a top-level resource that lists the buckets. It's reasonable 
to link to "the list of planets." Every planet is a resource: it's reasonable to link to 
"Venus." Every map of a planet is also a resource: it's reasonable to link to "the radar 
map of Venus." The list of planets is a resource of the first type, since there's only one 
of them. The planets and maps are also one-off resources: my service will serve a small 
number of maps for a small number of planets. 

Here are some of the resources so far: 

• The list of planets 

• Mars 

• Earth 

• The satellite map of Mars 

• The radar map of Venus 

• The topographic map of Earth 

• The political map of Earth 

But I can't just serve entire maps and let our clients figure out the rest. Then I'd just be 
running a hosting service for huge static map files: a RESTful service to be sure, but not 
a very interesting one. I must also serve parts of maps, oriented on specific points and 
places. 

Every point on a planet is potentially interesting, and so should be a resource. A point 
might represent a house, a mountain, or the current location of a ship. These are re- 
sources of the second type, because there are an infinite number of points on any planet. 
For every point on a planet there's a corresponding point on one or more maps. This 
is why I limited myself to addressable maps. When the map can be addressed by latitude 
and longitude, it's easy to turn a point on the planet into a point on a map. 

Here are some more of the resources so far: 

• 24.9195N 17.821E on Earth 

• 24.9195N 17.821E on the political map of Earth 

• 24.9195N 17.821EonMars 

• 44N OW on the geologic map of Earth 

I'll also serve places: points on a planet identified by name rather than by coordinates. 
My fantasy database contains a large but finite number of places. Each place has a type, 
a latitude and longitude, and each might also have additional associated data. For in- 
stance, an area of high pollution should "know" what pollutant is there and what the 
concentration is. As with points identified by latitude and longitude, the client should 
be able to move from a place on the planet to the corresponding point on any map. 



114 | Chapter5: Designing Read-Only Resource-Oriented Services 



Are Places Really Resources? 

Are places really resources of their own or are they just alternate names for the "point 
on a planet" resources I just defined? After all, every place is just a geographic point. 
Maybe I've got a situation where a single resource has two names: one based on latitude 
and longitude, and one based on a human-readable name. 

Well, consider a place representing the current location of a ship. It coincides with a 
specific point on the map, and it might be considered just an alternate name for that 
point. But in an hour, it'll coincide with a different point on the map. A business, too, 
might move over time from one point on the map to another. A place in this service is 
not a location: it's something that has location. A place has an independent life from 
the point on the map it occupies. 

This is analogous to the discussion in Chapter 4 about whether "version 1.0.3" and 
"the latest version" point to the same resource. It may happen that they point to the 
same data right now, but there are two different things there, and each might be the 
target of a hypertext link. I might link to one and say "Version 1.0.3 has a bug." I might 
link to the other and say "download the latest version. " Similarly, I might link to a point 
on Earth and say "The treasure is buried here." Or I might link to a place called "USS 
Mutiny" at the same coordinates and say "Our ship is currently here." Places are their 
own resources, and they're resources of the second type: each one corresponds to an 
object exposed through the service. 



I said earlier that place names are ambiguous. There are about 6,000 (an approxima- 
tion) cities and towns in the United States called Springfield. If a place name is unusual 
you can just say what planet it's on, and it's as good as specifying latitude and longitude. 
If a place name is common, you might have to specify more scoping information: giving 
a continent, country, or city along with the name of your place. Here are a few more 
sample resources: 

• The Cleopatra crater on Venus 

• The Ubehebe crater on Earth 

• 1005 Gravenstein Highway North, Sebastopol, CA 

• The headquarters of O'Reilly Media, Inc. 

• The place called Springfield in Massachusetts, in the United States of America, on 
Earth 

So far, this is pretty general stuff. Users want to know which maps we have, so we 
expose a one-off resource that lists the planets. Each planet is also a one-off resource 
that links to the available maps. A geographic point on a planet is addressable by lati- 
tude and longitude, so it makes sense to expose each point as an addressable resource. 
Every point on a planet corresponds to a point on one or more maps. Certain points 
are interesting and have names, so places on a planet are also accessible by name: a 
client can find them on the planet and then see that point on a map. 



Split the Data Set into Resources | 115 



All I've done so far is describe the interactions between parts of a predefined data set. 
I haven't yet exposed any algorithmically-generated resources, but it's easy enough to 
add some. The most common kind of algorithmic resource is the list of search results. 
I'll allow my clients to search for places on a planet that have certain names, or that 
match place-specific criteria. Here are some sample algorithmic resources: 

• Places on Earth called Springfield 

• Container ships on Earth 

• Craters on Mars more than 1 km in diameter 

• Places on the moon named before 1900 

Search results can be restricted to a particular area, not just a planet. Some more sample 
resources: 

• Places in the United States named Springfield 

• Sites of hot springs in Colorado 

• Oil tankers or container ships near Indonesia 

• Pizza restaurants in Worcester, MA 

• Diners near Mount Rushmore 

• Areas of high arsenic near 24.9195N 17.821E 

• Towns in France with population less than 1,000 

These are all algorithmically-generated resources, because they rely on the client pro- 
viding an arbitrary search string ("Springfield") or combining unrelated elements 
("Mount Rushmore" + diners, or "France" + towns + "population < 1000"). 

I could come up with new kinds of resources all day (in fact, that's what I did while 
writing this) . But all the resources I've thought up so far fit into five basic types, just 
enough to make the fantasy interesting. Example 5-1 gives the master list of resource 
types. 

Example 5-1. The five types of resources 

1. The list of planets 

2. A place on a planet — possibly the entire planet — identified by name 

3. A geographic point on a planet, identified by latitude and longitude 

4. A list of places on a planet that match some search criteria 

5. A map of a planet, centered around a particular point 

A real- life web service might define additional resources. Real web sites like Google 
Maps expose one obvious bit of functionality I haven't mentioned: driving directions. 
If I wanted to enhance my service I might expose a new algorithmically-generated re- 
source which treats a set of driving directions as a relationship between two places. The 



116 | Chapter5: Designing Read-Only Resource-Oriented Services 



representation of this resource might be a list of textual instructions, with references 
to points on a road map. 

General Lessons 

A RESTful web service exposes both its data and its algorithms through resources. 
There's usually a hierarchy that starts out small and branches out into infinitely many 
leaf nodes. The list of planets contains the planets, which contain points and places, 
which contain maps. The S3 bucket list contains the individual buckets, which contain 
the objects. 

It takes a while to get the hang of exposing an algorithm as a set of resources. Instead 
of thinking in terms of actions ("do a search for places on the map"), you need to think 
in terms of the results of that action ("the list of places on the map matching a search 
criteria"). You may find yourself coming back to this step if you find that your design 
doesn't fit HTTP's uniform interface. 

Name the Resources 

I've decided on five types of resources (see Example 5-1). Now they need names. Re- 
sources are named with URIs, so let's pick some. Remember, in a resource-oriented 
service the URI contains all the scoping information. Our URIs need to answer ques- 
tions like: "Why should the server operate on this map instead of that map?" and "Why 
should the server operate on this place instead of that place?" 

I'll root my web service at http://maps.example.com/. For brevity's sake I sometimes use 
relative URIs in this chapter and the next; understand that they're relative to http:// 
maps.example.com/. If I say /Earth/political, what I mean is http://maps.example.com/ 
Earth/political. 

Now let's consider the resources. The most basic resource is the list of planets. It makes 
sense to put this at the root URI, http://maps.example.com/. Since the list of planets 
encompasses the entire service, there's no scoping information at all for this resource 
(unless you count the service version as scoping information). 

For the other resources I'd like to pick URIs that organize the scoping information in 
a natural way. There are three basic rules for URI design, born of collective experience: 

1. Use path variables to encode hierarchy: /parent/child 

2. Put punctuation characters in path variables to avoid implying hierarchy where 
none exists: /parent/childl;child2 

3. Use query variables to imply inputs into an algorithm, for exam- 
ple: /search?q=jellyfish&start=20 



Name the Resources | 117 



Encode Hierarchy into Path Variables 

Let's make URIs for the second class of resource: planets and places on planets. There's 
one piece of scoping information here: what planet are we looking at? (Earth? Venus? 
Ganymede?) This scoping information fits naturally into a hierarchy: the list of planets 
is at the top, and underneath it is every particular planet. Here are the URIs to some of 
my planets. I show hierarchy by using the slash character to separate pieces of scoping 
information. 

• http://maps.example.com/Venus 

• http://maps.example.com/Earth 

• http://maps.example.com/Mars 

To identify geographical places by name I'll just extend the hierarchy to the right. You'll 
know you've got a good URI design when it's easy to extend hierarchies by tacking on 
additional path variables. Here are some URIs to various places on planets: 

• http://maps.example.com/Venus 

• http://maps.example.com/Venus/Cleopatra 

• http://maps.example.com/Earth/France/Paris 

• http://maps.example.com/Earth/Paris,%20France 

• http://maps.example.com/Earth/Little%20Rock,AR 

• http://maps.example.com/Earth/USA/Mount%20Rushmore 

• http://maps.example.com/Earth/1005%20Gravenstein%20Highway%20North, 
%20Sebastopol,%20CA%2095472 

We're now deep into web service territory. Sending a GET to one of these URIs invokes 
a remote operation that takes a variable number of arguments, and can locate a place 
on a planet to any desired degree of precision. But the URIs themselves look like normal 
web site URIs you can bookmark, cache, put on billboards, and pass to other services 
as input — because that's what they are. Path variables are the best way to organize 
scoping information that can be arranged hierarchically. The same structure you see in 
a filesystem, or on a static web site, can correspond to an arbitrarily long list of path 
variables. 

No Hierarchy? Use Commas or Semicolons 

The next resources I need to name are geographic points on the globe, represented by 
latitude and longitude. Latitude and longitude are tied together, so a hierarchy isn't 
appropriate. A URI like /Earth/24.9195/17.821 doesn't make sense. The slash makes 
it look like longitude is a subordinate concept to latitude, the way /Earth/Chicago sig- 
nals that Chicago is part of Earth. 



118 | Chapter5: Designing Read-Only Resource-Oriented Services 



Instead of using the slash to put two pieces of scoping information into a hierarchy, I 
recommend combining them on the same level of a hierarchy with a punctuation char- 
acter: usually the semicolon or the comma. I'm going to use a comma to separate lat- 
itude and longitude. This yields URIs like the following: 

• http://maps.example.com/Earth/24.91 95,17.821 

• http://maps. example. com/Venus/3, -80 

Latitude and longitude can also be used as scoping information to uniquely identify a 
named place. A human would probably identify Mount Rushmore as /Earth/USA/Mount 
%20Rushmore or as /vl/Earth/USA/SD/Mount%20Rushmore, but /vl/Earth/43.9;-103.46/ 
Mount%20Rushmore would be more precise. 

From a URI design perspective, the interesting thing here is that I'm stuffing two pieces 
of scoping information into one path variable. The first path variable denotes a planet, 
and the second one denotes both latitude and longitude. This kind of URI may look a 
little strange, because not many web sites or services use them right now, but they're 
catching on. 

I recommend using commas when the order of the scoping information is important, 
and semicolons when the order doesn't matter. In this case the order matters: if you 
switch latitude and longitude, you get a different point on the planet. So I used commas 
to separate the two numbers. It doesn't hurt that people already use commas in written 
language to separate latitude and longitude: URIs should use our existing conventions 
when possible. 

In another case the order might not matter. Consider a web service that lets you mix 
colors of paint to get the shade you want. If you're mixing red and blue paint, it doesn't 
matter whether you pour the red into the blue or the blue into the red: you get purple 
either way. So the URI /color-blends/red; blue identifies the same resource as /color - 
blends/blue; red. I think the semicolon is better than the comma here, because the order 
doesn't matter. This is just a typographical convention, but it helps a human being 
make sense of your web service URIs. The use of the semicolon feeds into an obscure 
idea called matrix URIs (http://www.w3.org/DesignIssues/MatrixURIs.html), a way of 
defining key-value pairs in URIs without using query variables. Some newer standards, 
like WADL, offer support for matrix URIs. They're especially useful if you ever need 
to put key-value pairs in the middle of a hierarchy. 



Name the Resources | 119 



URIs can become very long, especially when there's no limit to how deep 
you can nest the path variables. My web service might let clients name 
f {5J 1 a place using a lot of explicit scoping information: /Earth/North%20Amer 
ica/USA/California/Northern%20California/San%20Francisco%20Bay 
%20Area/Sebastopol/. . . 

The HTTP standard doesn't impose any restrictions on URI length, but 
real web servers and clients do. For instance, Microsoft Internet Ex- 
plorer can't handle URIs longer than 2,083 characters, and Apache 
won't respond to requests for URIs longer than 8 KBs. If some of your 
resources are only addressable given a great deal of scoping information, 
you may have to accept some of it in HTTP headers, or use overloaded 
POST and put scoping information in the entity-body. 



Map URIs 

Now that I've designed the URI to a geographic point on a planet, what about the 
corresponding point on a road map or satellite map? After all, the main point of this 
service is to serve maps. 

Earlier I said I'd expose a resource for every point on a map. For simplicity's sake, I'm 
not exposing maps of named places, only points of latitude and longitude. In addition 
to a set of coordinates or the name of a place, I need the name of the planet and the 
type of map (satellite map, road map, or whatever). Here are some URIs to maps of 
planets, places, and points: 

• http://maps.example.com/radar/Venus 

• http://maps.example.com/radar/Venus/65. 9,7. 00 

• http://maps.example.com/geologic/Earth/43. 9,-103. 46 

Scale 

A URI like /satellite/Earth/41., -112 says nothing about how detailed the map should 
be. I'm going to extend the first path variable so that it doesn't just specify the type of 
map: it can also specify the scale. I'll expose a very small-scale map at /satellite. 10/ 
Earth, a very large-scale map at /satellite. l/Earth, and maps of other scales in be- 
tween. I'll choose a sensible default scale: probably a large scale like 2. Here are some 
possible URIs for the same map at different scales: 

• /satellite. lo/Earth/41, -112: 1:24,000; 2,000 feet to the inch. A map for hiking 
or prospecting. Centered on 41°N 1 12°W on Earth, this map would show the banks 
of Utah's Great Salt Lake. 

• /satellite. 5/Earth/41, -112: 1:250,000; 4 miles to the inch. The scale of a highway 
map. Centered on 41°N 1 12°W, this map would show the northern suburbs of Salt 
Lake City. 



120 I Chapter 5: Designing Read-Only Resource-Oriented Services 



• /satellite. l/Earth/41, -112: 1:51,969,000; 820 miles to an inch. (That's 820 
miles/inch at the equator. At this scale, the curvature of the earth distorts the scale 
of a 2D map.) The scale of a world map. Centered on41°N 112°W, this map would 
show much of Utah and surrounding states. 

The scale affects not only the natural size of the map in pixels, but which features are 
shown. A small town would be represented in fair detail on a map at scale 10, but would 
only be a point at scale 5 if it showed up at all. 

How did I decide that scale l would be a large-scale map, and scale 10 would be a small- 
scale map? Why not the reverse? I used a common technique for URI design. I exag- 
gerated the decision I was making, figured out how the generalized situation should 
work, and then scaled my decision back down. 

Maps can always get more detailed, but there's a limit how small they can get. # If I 
decide to acquire some new data for my map service, I'd never buy a map that shows 
the world in less detail than the world map at scale l. There'd be no point. However, 
it's quite possible that I'll find maps that are more detailed than the one at scale 10. 
When I find those maps, I can make them available through my service and assign them 
scales of 11, 12, and so on. If I'd assigned the most detailed map a scale of 1, I'd have 
to assign scales of 0, -1, and so on to any new maps. The URIs would look strange. 
This means larger numbers make good URIs for more detailed maps. I may never ac- 
tually get those more detailed maps, but thinking about them revealed a truth about 
my URI design. 

Algorithmic Resource? Use Query Variables 

Most web applications don't store much state in path variables: they use query variables 
instead. You may have seen URIs like this: 

• http://www.example.com/colorpairPcolorl =red&color2=blue 

• http:llwww. example, com/ articles ?start=2 0061201 &end=2 0071201 

• http:llwww. example. com/weblog?post=My-Opinion-About-Taxes 

Those URIs would look better without the query variables: 

• http-.llwww. example. com/colorpair/red;blue 

• http-.llwww. example, com/ articles/2 0061201-20071201 

• http://www.example.com/weblog/My-Opinion-About-Taxes 

Sometimes, though, query variables are appropriate. Here's a Google search URI: http:// 
www. google. com/search ?q=jelly 'fish. If the Google web application used path variables, 
its URIs would look more like directories and less like the result of running an algo- 
rithm: http://www.google.com/search/jellyfish. 

* Up to a point, anyway. See On Exactitude in Science by Jorge Luis Borges. 



Name the Resources | 121 



Both of those URIs would be legitimate resource-oriented names for the resource "a 
directory of web pages about jellyfish." The second one doesn't look quite right, 
though, because of how we're socialized to look at URIs. Path variables look like you're 
traversing a hierarchy, and query variables look like you're passing arguments into an 
algorithm. "Search" sounds like an algorithm. For example, http://www.google.com/ 
directory/jellyfish" might work better than /search/jellyfish. 

This perception of query variables is reinforced whenever we use the Web. When you 
fill out an HTML form in a web browser, the data you input is turned into query var- 
iables. There's no way to type "jellyfish" into a form and then be sent to http://www.goo 
gle.com/search/jellyfish. The destination of an HTML form is hard-coded to http:// 
www.google.com/search/, and when you fill out that form you end up at http://www.goo 
gle.com/search?q=jellyfish. Your browser knows how to tack query variables onto a base 
URL It doesn't know how to substitute variables into a generic URI like http://www.goo 
gle. com/ search/ {a}. 

Because of this precedent, a lot of REST-RPC hybrid services use query variables when 
it would be more idiomatic to use path variables. Even when a hybrid service happens 
to expose resources RESTfully, the resources have URIs that make them look 
like function calls: URIs such as http:llapi.jlickr.com/serviceslr est/ '?method=flickr.pho 
tos.search&tags=penguin. Compare that URI to the corresponding URI on the human- 
usable Flickr site: http://flickr.com/photos/tags/penguin. 

I've managed to avoid query variables so far: every planet, every point on a planet, and 
every corresponding map is addressable without them. I don't really like the way query 
variables look in a URI, and including them in a URI is a good way to make sure that 
URI gets ignored by tools like proxies, caches, and web crawlers. Think back to the 
Google Web Accelerator I mentioned in "Why safety and idempotence matter" in 
"Split the Data Set into Resources. It never pre-fetches a URI that includes a query 
variable, because that's the kind of URI exposed by poorly-designed web applications 
that abuse HTTP GET. My service won't abuse GET, of course, but outside applica- 
tions have no way of knowing that. 

But I've got one more type of resource to represent — lists of search results — and I'm 
out of tricks. It doesn't make sense to keep going down the hierarchy of place, and I 
can't keep piling on punctuation just to avoid the impression that my service is running 
an algorithm. Besides, this last type of resource is the result of running an algorithm. 
My search algorithm finds places that match map-specific criteria, just as a search en- 
gine finds web sites that match the client's keywords. Query variables are perfectly 
appropriate for naming algorithmic resources. 

The search interface for places can get as complex as I need it to be. I could expose a 
name query variable for place names and pollutant for sites of high pollution and cui 
sine for restaurants and all sorts of other query variables. But let's imagine I've got the 
technology to make it simple. The only query variable I'll add is show, which lets the 
client specify in natural language what feature(s) they're searching for. The server will 



122 | Chapter 5: Designing Read-Only Resource-Oriented Services 



parse the client's values for show and figure out what places should be in the list of 
search results. 

In "Split the Data Set into Resources" earlier in this chapter, I gave a whole lot of sample 
search resources: "places on Earth called Springfield," and so on. Here's how a client 
might use show to construct URIs for some of those resources. 

• http://ma.ps. example. com/Earth?show=Springfield 

• http:llmaps. example. com/Mars ?show=craters+bigger+than+ 1km 

• http://ma.ps. example. com/Earth/Indonesia?show=oil+tankers&show=container 
+ships 

• http://maps.example.com/Earth/USA/Mount%20Rushmore?show=diners 

• http://maps.example.com/Earth/24.9195;! 7.821 ?show=arsenic 

Note that all of these URIs are searching the planet, not any particular map. 

URI Recap 

That's a lot of details. After all, this is the first place where my fantasy resources come 
into contact with the real world of HTTP. Even so, my service only supports three basic 
kinds of URI. To recap, here they are: 

• The list of planets: /. 

• A planet or a place on a planet: /{planet}/[{scoping-information}/][{place- 
name}]: The value of the optional variable {scoping-information} will be a hierar- 
chy of place names like /USA/New%20England/Maine/ or it will be a latitude/longitude 
pair. The value of the optional variable {name} will be the name of the place. 

This type of URI can have values for show tacked onto its query string, to search 
for places near the given place. 

• A map of a planet, or a point on a map: / '{map-type}{scale} / '{planet} /[{scoping- 
information}]. The value of the optional variable {scoping-information} will al- 
ways be a latitude/longitude pair. The value of the optional variable {scale} will 
be a dot and a number. 

Design Your Representations 

I've decided which resources I'm exposing, and what their URIs will look like. Now I 
need to decide what data to send when a client requests a resource, and what data 
format to use. This is just a warmup, since much of Chapter 9 is devoted to a catalog 
of useful representation formats. Here, I have a specific service in mind, and I need to 
decide on a format (or a set of formats) that can meet the goals of any RESTful repre- 
sentation: to convey the current state of the resource, and to link to possible new ap- 
plication and resource states. 



Design Your Representations | 123 



The Representation Talks About the State of the Resource 

The main purpose of any representation is to convey the state of the resource. Remem- 
ber that "resource state" is just any information about the underlying resource. In this 
case, the state is going to answer questions like: what does this part of the world look 
like, graphically? Where exactly is that meteor crater, in latitude and longitude? Where 
are the nearby restaurants and what are their names? Where are the container ships 
right now? Representations of different resources will represent different items of state. 

The Representation Links to Other States 

The other job of the representation is to provide levers of state. A resource's represen- 
tation ought to link to nearby resources (whatever "nearby" means in context) : possible 
new application states. The goal here is connectedness: the ability to get from one re- 
source to another by following links. 

This is how web sites work. You don't surf the Web by typing in URIs one after the 
other. You might type in one URI to get to a site's home page, but then you surf by 
following links and filling out forms. One web page (a "state" of the web site) contains 
links to other, related web pages (nearby "states"). 

Of course, a computer program can't look at a document and decide which links it 
wants to follow. It only has the wants the programmer gives it. If a web service includes 
links in its representations, the representations must also contain machine-readable 
signals as to where each link leads. A programmer can write his or her client to pick up 
on those signals and decide which link matches up with the goals of the moment. 

These links are the levers of application state. If a resource can be modified with PUT, 
or it can spawn new resources in response to POST, its representation ought to also 
expose the levers of resource state. The representation ought to provide any necessary 
information about what the POST or PUT request should look like. I'm getting a little 
ahead of myself here, since all the resources in this chapter are read-only. For now, I'll 
be creating representations that expose the levers of application state. 

Representing the List of Planets 

The "home page" of my map service is a good place to start, and a good place to 
introduce the issues behind choosing a representation format. Basically, I want to dis- 
play a list of links to the planets for which I have maps. What's a good format for a 
representation of a list? 

There's always plain text. This representation in Example 5-2 shows one planet per 
line: the URI and then the name. 



124 | Chapter 5: Designing Read-Only Resource-Oriented Services 



Example 5-2. A plain-text representation of the planet list 
http : / /maps . example . com/Earth Earth 
http : //maps . example . com/Venus Venus 

This is simple but it requires a custom parser. I generally think a structured data format 
is better than plain text, especially as representations get more complex. (Of course, if 
plain text is what you're serving, there's no need to dress it up as something else.) JSON 
keeps the simplicity of plain text but adds a little bit of structure (see Example 5-3). 

Example 5-3. A ]SON representation of the planet list 

[{url="http: //maps. example. com/Earth, description="Earth"}, 
{url="http: //maps. example. com/Venus, description="Venus"}, 
...] 

The downside is that neither JSON nor plain text are generally considered "hyperme- 
dia" formats. Another popular option is a custom XML vocabulary, either with or 
without a schema definition (see Example 5-4). 

Example 5-4. An XML representation of the planet list 
<?xml version="l.O" standalone='yes' ?> 
<planets> 

<planet href="http://maps. example. com/Earth" name="Earth" /> 
<planet href="http://maps. example. com/Venus" name="Venus" /> 

</maps> 

These days, a custom XML vocabulary seems to be the default choice for web service 
representations. XML is excellent for representing documents, but I think it's actually 
pretty rare that you would have to come up with a custom vocabulary. The basic prob- 
lems have already been solved, and most of the time you can reuse an existing XML 
vocabulary. As it happens, there's already an XML vocabulary for communicating lists 
of links called Atom. 

I cover Atom in detail in Chapter 9. Atom will work to represent the list of planets, but 
it's not a very good fit. Atom is designed for lists of published texts, and most of its 
elements don't make sense in this context — what does it mean to know the "author" 
of a planet, or the date it was last modified? Fortunately, there's another good XML 
language for displaying lists of links: XHTML. Example 5-5 shows one more represen- 
tation of the planet list, and this is the one I'm actually going to use. 

Example 5-5. An XHTML representation of the planet list 
<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 

<head> 

<title>Planet List</title> 
</head> 
<body> 



Design Your Representations | 125 



<ul class="planets"> 
<lixa href="/Earth">Earth</ax/li> 
<lixa hre-f="/Venus">Venus</ax/li> 

</ul> 

</body> 
</html> 

It might seem a little odd to use XHTML, a technology associated with the human web, 
as a representation format for a web service. I chose it for this example because HTML 
solves many general markup problems and you're probably already familiar with it. I'd 
probably choose it for a real web service, for exactly the same reasons. Though it's 
human-readable and easy to render attractively, nothing prevents well-formed HTML 
from being processed automatically like XML. XHTML is also extensible. I turned a 
generic XHTML list into a list of "planets" using XHTML's class attribute. This is a 
simple example of an XHTML microformat: a way of adding semantic meaning to 
XHTML's markup tags. I cover some standard microformats in Chapter 9. 

Representing Maps and Points on Maps 

What about the maps themselves? What do I serve if someone asks for a satellite map 
of the Moon? The obvious thing to send is an image, either in a traditional graphics 
format like PNG or as a SVG scalar graphic. Except for the largest-scale maps, these 
images will be huge. Is this OK? It depends on the audience for my web service. 

If I'm serving clients with ultra-high bandwidth who expect to process huge chunks of 
map data, then huge files are exactly what they want. But it's more likely my clients 
will be like the users of existing map applications like Google and Yahoo! Maps: clients 
who want smaller-sized maps for human browsing. 

If the client asks for a medium-scale hiking map centered around 43N 71W, it's surely 
a waste of bandwidth to send a map of the whole world centered around that point. 
Instead I should send a little bit of a hiking map, centered around that point, along with 
navigation links that let the client change the focus of the map. Even if the client asks 
for a detailed map of the whole world, I don't need to send the entire map: I can send 
part of the map and let the client fetch the rest as needed. 

This is more or less how the online map sites work. If you visit http://maps.goo 
gle.com/, you get a political map centered on the continental United States: that's its 
representation of "a map of Earth." If you visit http://ma.ps. google. com/maps ?q=New 
+Hampshire, you get a road map centered on Concord, the capital city. In either case, 
the map is divided into square "tile" images 256 pixels on a side. The client (your web 
browser) fetches tiles as needed and stitches them together to form a navigable map. 

Google Maps splits the globe into a grid of 256-pixel square tiles, pretty much ignoring 
issues of latitude and longitude, and generates static images for each tile. It does this 



126 | Chapter 5: Designing Read-Only Resource-Oriented Services 



10 times, once for every zoom level. This is efficient (though it does use a lot of storage 
space), but for pedagogical purposes I've chosen a conceptually simpler system. I'm 
assuming my map service can dynamically generate and serve a 256 x256 image at any 
scale, centered on any point of latitude and longitude on any map. 

• > Google Maps's static tile system is more complex because it adds an- 

other coordinate system to the map. Besides latitude and longitude, you 
f[v can also refer to a place by which tile it's on. This makes the navigation 
representation simpler, at the expense of complicating the design. 

When the client requests a point on a map, I'll serve a hypermedia file that includes a 
link to a tiny map image (a single, dynamically-generated tile) centered on that point. 
When the client requests a map of an entire planet, I'll pick a point on that planet 
somewhat arbitrarily and serve a hypermedia file that links to an image centered on 
that point. These hypermedia files will include links to adjacent points on the map, 
which will include more links to adjacent points, and so on. The client can follow the 
navigation links to stitch many tiles together into a map of any desired size. 

So Example 5-6 is one possible representation of http://maps.example.com/road/ 
Earth. Like my representation of the list of planets, it uses XHTML to convey resource 
state and to link to "nearby" resources. The resource state here is information about a 
certain point on the map. The "nearby" resources are nearby in a literal sense: they're 
nearby points. 

Example 5-6. An XHTML representation of the road map of Earth 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 

<head> 

<title>Road Map of Earth</title> 
</head> 
<body> 

<img class="map" src="/road.2/Earth/images/37.0,-95.png" alt="Map tile"/> 

<a class="map_nav" href="46. 0518,-95. 8">North</a> 

<a class="map_nav" href="4l.3776, -89.7698">Northeast</a> 

<a class="map_nav" href="36. 4642,-84. 5l87">East</a> 

<a class="map_nav" href="32.35l3,-90.4459">Southeast</a> 

<a class="zoom_in" href="/road.l/Earth/37.0;-95.8">Zoom out</a> 
<a class="zoom_out" href="/road.3/Earth/37.0;-95.8">Zoom in</a> 

</body> 
</html> 



Design Your Representations | 127 



Worm- 

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ouri 



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Figure 5-1. A representation for /road/Earth.3/images/37. 0,-95. png 

Now when a client requests the resource "a road map of Earth" at the URI /road/ 
Earth, the representation they get is not an enormous, insanely detailed image that they 
can't deal with. It's a small XHTML document, one that includes links to several other 
resources. 

A human being can just look at this document and know what it means. A computer 
program doesn't have that ability; it has to be programmed in advance by someone 
who can think about a whole class of these documents and write code to find which 
bits have the meaning. A web service works by making promises that it will serve rep- 
resentations of resources with a certain structure. That's why my representation is full 
of semantic cues like "zoom_in" and "Northeast". Programmers can write clients that 
pick up on the semantic cues. 



Representing the Map Tiles 

The representation of a road map of Earth, given in Example 5-6, has a lot of links in 
it. Most of these are links to XHTML documents that look a lot like "a road map of 
Earth" does: they're representations of points on the map at various zoom levels. The 
most important link, though, is the one in the IMG tag. That tag's src attribute references 
the URI http://maps. example. com/road/Earth.8/images/37.0;-95. png. 

This is a new kind of resource, and I haven't really considered it before, but it's not 
hard to figure out what it is. This resource is "an image centered around 37°N 95.8°W 
on the road map of Earth." In my service, the representation oithat resource will be a 
256 x256 image showing the geographic center of the continental U.S. (see Figure 5-1). 

The image in Figure 5-1 is 256 pixels square, and represents an area of the Earth about 
625 miles square. This image is distinct from the representation of "39°N 95.8°W on 
the road map of Earth." that would be an XHTML file like the one in Example 5-6. The 



128 | Chapter 5: Designing Read-Only Resource-Oriented Services 




Figure 5-2. A representation for /road.8/Earth/images/37. 0,-95. png 



XHTML file would include this image by reference, and also link to a lot of nearby 
points on the map. 

Here's another example: if the client requests /road.8/Earth/32.37, -86.30, my service 
will send an XHTML representation whose IMG tag references /road.8/Earth/images/ 
32. 37, -86. 30. png (see Figure 5-2). This is a very detailed road map centered on 
32.37°N, 86.30°W on Earth. 

That image too is 256 pixels square, but it represents an area of the Earth only a half- 
mile square. Scale makes the difference. 

The important thing here is not the exact setup of the tile system or the precise format 
of the URIs to the tile resources. What's important is what I'm putting into my repre- 
sentations. The URI /road/Earth refers to a resource: "a road map of Earth". You'd 
expect a pretty big image as the representation of that resource. You'd at least expect 
one that showed all of Earth. But my service sends an XHTML document that references 
a 256x256 tile image that doesn't even cover four U.S. states. How can that document 
be a good representation for "a road map of Earth"? 

A representation conveys the state of its resource, but it doesn't have to convey the 
entire state of the resource. It just has to convey some state. The representation of 
"Earth" (coming up in a bit) isn't the actual planet Earth, and the representation of "a 
road map of the Earth" can reference just a simple image tile. But this representation 
does more than that: the XHTML file links this arbitrarily chosen point on the map to 
other nearby points on the part of the map directly to the north of this tile, the part 
directly to the east, and so on. The client can follow these links to other resources and 
piece together a larger picture. The map is made of a bunch of connected resources, 
and you can get as many graphical tiles as you need by following the links. So in a sense, 
this representation does convey all the state there is about the road map of Earth. You 
can get as much of that state as you want by following its links to other resources. 



Design Your Representations | 129 



It's worth repeating here that if my clients actually need detailed multigigabyte maps, 
there's no point in me chopping up the state of the map into these tiny tiles. It'd be 
more efficient to have the representation of /road/Earth?zoom=l convey the entire state 
of the map with one huge image. I've designed for clients that only really want part of 
a map, and wouldn't know what to do with one huge map of the earth if I gave it to 
them. The clients I have in mind can consume the XHTML files, load the appropriate 
images, and automatically follow links to stitch together a map that's as big as neces- 
sary. You could write an Ajax client for my web service that worked like the Google 
Maps application. 

Representing Planets and Other Places 

I've shown representations for the planet list, for maps of the planets, and for points 
on the maps. But how are you supposed to get from the planet list to, say, the road map 
of Earth? Presumably you click "Earth" in the planet list, sending a GET request 
to /Earth, and get back a representation of Earth. This representation includes a bunch 
of links to maps of Earth. At this point you follow a second link to the road map of 
Earth. Well, I just described the representation of Earth. My representation of a planet 
contains whatever useful information I have about the planet, as well as a set of links 
to other resources: maps of the planet (see Example 5-7). 

Example 5-7. An XHTML representation of a place: the planet Earth 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/i999/xhtml" xml:lang="en"> 

<headxtitle>Earth</titlex/head> 
<body> 

<dl class="place"> 
<dt>name</dt> <dd>Earth</dd> 
<dt>maps</dt> 
<dd> 

<ul class="maps"> 

<lixa class="map" href="/road/Earth">Road</ax/li> 

<lixa class="map" href="/satellite/Earth">Satellite</a> 

</ul> 
</dd> 

<dt>type</dt> <dd>planet</dd> 
<dt>description</dt> 
<dd> 

Third planet from Sol. Inhabited by bipeds so amazingly primitive 
that they still think digital watches are a pretty neat idea. 
</dd> 
</dl> 

</body> 
</html> 



130 I Chapter 5: Designing Read-Only Resource-Oriented Services 



I've chosen to represent places as lists of key-value pairs. Here, the "place" is the planet 
Earth itself. Earth in this system is a named place, just like San Francisco or Egypt. I'm 
representing it using the dd tag: HTML's standard way of presenting a set of key-value 
pairs. Like any place, Earth has a name, a type, a description, and a list of maps: links 
to all the resources that map this place. 

Why am I representing a planet as a place? Because now my clients can parse the rep- 
resentation of a planet with the same code they use to parse the representation of a 
place. Example 5-8 is a representation for Mount Rushmore on Earth. You might get 
this XHTML file back in response to a GET request for /Earth/USA/Mount%20Rushmore. 

Example 5-8. An XHTML representation of a place: Mount Rushmore 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 

<headxtitle>Mount Rushmore</titlex/head> 
<body> 

<ul class="places"> 
<li> 

<dl class="place"> 
<dt>name</dt> <dd>Mount Rushmore</dd> 
<dt>location</dt> 
<dd> 

<a class="coordinates" href="/Earth/43.9;-95.9">43.9°N 95.8°W</a> 

</dd> 
<dt>maps</dt> 

<ul class="maps"> 
•clixa class="map" href="/road/Earth/43.9;-95.9">Road</ax/dd> 
<lixa class="map" href="/satellite/Earth/43.9;-95.9">Satellite</a> 

</ul> 
</dd> 

<dt>type</dt> <dd>monument</dd> 
<dt>description</dt> 
<dd> 

Officially dedicated in 1991. Under the jurisdiction of the 

<a href="http://www.nps.gov/">National Park Service</a>. 
</dd> 
</dl> 
</li> 

</body> 
</html> 

Rather than serve a map image of Mount Rushmore, or even an XHTML page that links 
to that image, this representation links to resources I've already defined: maps of the 
geographical point where Mount Rushmore happens to be located. Those resources 
take care of all the imagery and navigation details. The purpose of this resource is to 
talk about the state of the place, and what it looks like on a map is just one bit of that 



Design Your Representations | 131 



state. There's also its name, its type ("monument"), and its description. The only dif- 
ference between the representation of a planet and that of a place is that a place has a 
location in its definition list, and a planet doesn't. A client can parse both representa- 
tions with the same code. 

You may also have noticed that you don't have to write a special client for this web 
service at all. You can use a plain old web browser. Starting at the home page (http:// 
maps.example.com/), you click a link ("Earth") to select a planet. You get the repre- 
sentation shown in Example 5-7, and you click "Road" to see a road map of Earth. 
Then you navigate that map by clicking links ("North," "Zoom out"). My web service 
is also a web site! It's not a very pretty web site, because it's designed to be used by a 
computer program, but nothing prevents a human from consuming it (or debugging 
it) with a web browser. 

If you get only one thing out of this book, I hope it's that this idea starts seeming natural 
to you (assuming it didn't before). Web services are just web sites for robots. My map 
service is particularly web site-like: it connects its resources together with hypermedia, 
the hypermedia representations happen to be HTML documents, and (so far) it doesn't 
use any features that web browsers don't support. But all RESTful resource-oriented 
web services partake of the nature of the Web, even if you can't use them with a standard 
web browser. 

Example 5-9 shows one more representation: the representation of a point on the map. 

Example 5-9. An XHTML representation of the point 43.9°N 1 03.46°W on Earth 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 

<head> 

<title>43.9°N 103.46°W on Earth</title> 
</head> 
<body> 

<P> 

Welcome to 

<a class="coordinates" href="/Earth/43.9;-l03.46">43.9°N 
103.46°W</a> 
on scenic <a class="place" href="/Earth">Earth</a>. 
</p> 

<p>See this location on a map:</p> 

<ul class="maps"> 
<lixa class="map" href="/road/Earth/43.9;-95.9">Road</ax/li> 
<lixa class="map" href="/satellite/Earth/43.9;-95.9">Satellite</ax/li> 

</ul> 

<p>Things that are here:</p> 



132 I Chapter 5: Designing Read-Only Resource-Oriented Services 



<ul class="places"> 
<lixa hre-f="/Earth/43.9;-95.9/Mount%20Rushmore">Mount Rushmore</ax/li> 
</ul> 

<form id="searchPlace" method="get" action=""> 
<p> 

Show nearby places, features, or businesses: 
<input name="show" repeat="template" /> <input class="submit" /> 
</p> 
</form> 

</body> 
</html> 

This representation consists entirely of links: links to maps centered around this point, 
and links to places located at this point. It has no state of its own. It's just a gateway to 
other, more interesting resources. 

Representing Lists of Search Results 

I've shown representations for the planet list, for a planet, for points and places on a 
planet, and for the maps themselves. What about my algorithmic resources, the search 
results? What's a good representation of the resource "diners near Mount Rushmore" 
(/Earth/USA/Mount%20Rushmore?show=diners)? What about "Areas of high arsenic near 
24.9195°N 17.821°E" (/Earth/24. 9l95;l7.82l?show=arsenic)? 

A list of search results is of course associated with the place being "searched," so a 
representation of "diners near Mount Rushmore" should link to the place "Mount 
Rushmore." That's a start. 

When the client searches in or around a place, they're searching for more places. 
Whether the search string is an ambiguous place name ("Springfield") or a more general 
description of a place ("diners," "arsenic"), the results will be places on the map: cities 
named Springfield, diners, or sites with high arsenic readings. So a list of search results 
takes one place ("Mount Rushmore"), and associates certain other places ("Joe's Din- 
er") with it. 

A list of search results, then, can be nothing but a list of links to resources I've already 
defined: named places on the map. If the client is interested in a place, it can follow the 
appropriate link and find out about its state. 

Example 5-10 shows the representation of a set of search results. The search is an 
attempt to find places called "Springfield" in the United States: its URI would 
be /Earth/USA?show=Springfield. 

Example 5-10. The representation of "a list of places called Springfield in the United States" 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : //www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/i999/xhtml" xml:lang="en"> 



Design Your Representations | 133 



<headxtitle>Search results: "Springfield"</titlex/head> 
<body> 

<P> 

Places matching <span class="searchterm">Springfield</span> 
in or around 

<a class="place" href="/Earth/USA">the United States of America</a>: 
</p> 

<ul> 
<li> 

<a class="place" href="/Earth/USA/IL/Springfield">Springfield, IL</a> 
</li> 
<li> 

<a class="place" href="/Earth/USA/MA/Springfield">Springfield, MA</a> 
</li> 
<li> 

<a class="place" href="/Earth/USA/MO/Springfield">Springfield, M0</a> 
</li> 

</body> 
</html> 

This representation is made up almost entirely of links to places. There's the link to the 
place that was searched, "the United States of America" (a place of type "country"). 
There are also links to various places that matched the search criteria. Each of these 
places is a resource, and exposes a representation that looks like Example 5-8. Each 
link contains enough scoping information to uniquely identify the Springfield in 
question. 

A client can follow the links in the representations to find information about the places, 
as well as maps of them. Figure 5-3 shows some of the links you can follow 
from /Earth/USA?show=Springfield. 

Google Maps presents search results by sewing image tiles together to make a large- 
scale map, and annotating the map with graphical markers. You can write a client for 
this web service that does the same thing. The first step is to build the large-scale map. 
The client follows the initial link to /Earth/USA and gets a representation like the one 
in Example 5-4. This gives the client the address of one graphical tile. The client can 
get adjacent tiles by following navigation links, and stitch them together into a large- 
scale tile map of the whole country. 

The second step is to stick markers on the map, one for each search result. To find out 
where a marker should go, the client follows one of the search result links, fetching a 
representation like the one in Example 5-8. This representation lists the latitude and 
longitude of the appropriate Springfield. 

That's potentially a lot of link following, but my representations are simple and so are 
the rules for going from one to another. I've spread out my data set over a huge number 
of resources, and made it easy to find the resource you want by following links. This 
strategy works on the human web, and it works on the programmable web too. 



134 | Chapter 5: Designing Read-Only Resource-Oriented Services 




Figure 5-3. Where can you go from the list of search results? 



Link the Resources to Each Other 

Since I designed all my resources in parallel, they're already full of links to each other 
(see Figure 5-3). A client can get the service's "home page" (the planet list), follow a 
link to a specific planet, follow another link to a specific map, and then follow naviga- 
tion and zoom links to jump around the map. A client can do a search for places that 
meet certain criteria, click one of the search results to find out more about the place, 
then follow another link to locate the place on a map. 

One thing is still missing, though. How is the client supposed to get to a list of search 
results? I've set up rules for what the URI to a set of search results looks like, but if 
clients have to follow rules to generate URIs, my service isn't well connected. 

I want to make it possible for a client to get from /Earth/USA/Mount%20Rushmore 
to /Earth/USA/Mount%20Rushmore?show=diners. But it does no good to link to "diners" 
specifically: that's just one of infinitely many things a client might search for. I can't 
put infinitely many links in the representation of /Earth/USA/Mount%20Rushmore just in 
case someone decides to search for pet stores or meteor craters near Mount Rushmore. 

HTML solves this problem with forms. By sending an appropriate form in a represen- 
tation, I can tell the client how to plug variables into a query string. The form represents 
infinitely many URIs, all of which follow a certain pattern. I'm going to extend my 
representations of places (like the one in Example 5-8) by including this HTML form 
(see Example 5-11). 



Link the Resources to Each Other | 135 




Places on Earth 
called "Mount Rushmore" 



Link 




Figure 5-4. The path from the service root to a diner near Mount Rushmore 



Example 5-11. An HTML form for searching a place 

<form id="searchPlace" method="get" action=""> 
<p> 

Show places, features, or businesses: 
<input id="term" repeat="template" name="show" /> 
<input class="submit" /> 
</p> 

</formx/screen> 

A person using a web browser would see this form as a set of GUI elements: a text box, 
a button, and a set of labels. They'd put some data into the text box, click the button, 
and be taken to another URI. If they were at /Earth/USA/Mount%20Rushmore, and they'd 
typed in "diners," their web browser would make a GET request to /Earth/USA/Mount 
%20Rushmore?show=diners. An automatic client can't display the form to a human, but 
it would work the same way. Given a preprogrammed desire to search a place, it would 
look for the searchPlace form and use the form definition as a guide to constructing 
the URI /Earth/USA/Mount%20Rushmore?show=diners. 



You probably haven't seen the repeat="template" syntax before. It's a 
feature of XHTML 5, which is still being designed as this book goes to 
I press. Occasionally in this chapter and the next, I'll introduce a feature 
of XHTML 5 to work around the shortcomings of XHTML 4 as a hy- 
permedia format. 

The problem here is that my service accepts any number of values for 
the query variable show. A client can make a simple search such 
as ?show=diners or perform a complicated search such 
as ?show=diners&show=arsenic&show=towns&show=oil+tankers. 

A form in XHTML 4 could allow the latter request if it showed four text 
boxes, all called show. But an HTML form can never show an arbitrary 
number of text boxes, which is what I need to truly capture the capa- 
bilities of my service. XHTML 5 has a feature called the repetition mod- 
el, which allows me to express an arbitrary number of text boxes without 
writing an infinitely long HTML page. 



136 | Chapter 5: Designing Read-Only Resource-Oriented Services 



Now my service is better connected. It's now possible to get from the list of planets to 
a description of a diner near Mount Rushmore (assuming there is one). Figure 5-4 
illustrates the journey. Starting at the service root (/), the client selects the planet Earth 
(/Earth). The client uses the HTML form in that representation to search for places 
called "Mount Rushmore" on Earth (/Earth?show=Mount%20Rushmore). Hopefully the 
top search result will be Mount Rushmore itself, and the client can follow the first 
search result link to /Earth/USA/Hount%20Rushmore. The representation of Mount Rush- 
more has a search form too, and the client enters "diners" in that. Assuming there are 
any nearby diners, the client can follow the first search result link to find a diner near 
Mount Rushmore. 

The search function doesn't just keep clients from having to mint their own URIs. It 
resolves human place names, which are always fuzzy, into canonical resource URIs. A 
client should be also be able to search for "Mount Rushmore National Monument" 
and get /Earth/USA/Mount%20Rushmore as a search result, just like the client can search 
for "Springfield" and pick which Springfield they mean. This is a useful feature for any 
client that lets users type in their own place names. 

The HTTP Response 

I'm almost done with this design. I know what data I'm serving. I know which HTTP 
requests clients will send when asking for the data. I know how the data will be repre- 
sented as I serve it. I still have to consider the HTTP response itself. I know what my 
possible representations will look like, and that's what's going in the entity-body, but 
I haven't yet considered the possible response codes or the HTTP headers I'll send. I 
also need to think about possible error conditions: cases where my response signals an 
error instead of delivering a representation. 

What's Supposed to Happen? 

This step of the design is conceptually simple, but it's the gateway to where you're 
going to spend much of your implementation time: making sure that client requests 
are correctly turned into responses. 

Most read-only resources have a pretty simple typical course of events. The user sends 
a GET request to a URI, and the server sends back a happy response code like 200 
("OK"), some HTTP headers, and a representation. A HEAD request works the same 
way, but the server omits the representation. The only main question is which HTTP 
headers the client should send in the request, and which ones the server should send 
in the response. 

The HTTP response headers are a fairly large toolkit, and most of them don't apply to 
this simple service. (For descriptions of the standard HTTP headers, see Appendix C.) 
In my service, the main HTTP response header is Content-Type, which tells the client 
the media type of the representation. My media types are application/xhtml+xml for 



The HTTP Response | 137 



the map representations and search results, and image/png for the map images. If you've 
done any server-side web programming you already know about Content-Type: every 
HTTP server and framework uses it. 

I don't use HTTP request headers very often. I think it's best if the client can tweak the 
representation by tweaking the URI to the resource, rather than tweaking the request 
headers. But there is one set of headers that I think ought to be built into every HTTP 
client, every web service, and every service hacker's brain: the ones that make condi- 
tional GET possible. 

Conditional HTTP GET 

Conditional HTTP GET saves client and server time and bandwidth. It's implemented 
with two response headers (Last-Modified and ETag), and two request headers (If- 
Modified-Since and If-None-Match). 

I cover conditional GET in detail in Chapter 8, but the discussion there is somewhat 
detached from specific services. This discussion is tied to the map service, and covers 
just enough to get you thinking about conditional GET as you design your services. 

Certain resources are likely to be very popular: "A road map of the United States," "a 
satellite map of Earth," or "restaurants in New York City." A single client is likely to 
make a request for certain resources many times over its lifespan. 

But this data is not constantly changing. Map data stays pretty constant over time. 
Satellite imagery is updated every few months at most. Restaurants come and go, but 
not on a minute-by-minute basis. Only a few resources are based on data that's con- 
stantly changing. Most of the time, the client's second and subsequent HTTP requests 
for a resource are wasted. They could have just reused the representation from their 
first request. But how are they supposed to know this? 

This is where conditional GET comes in. Whenever a server serves a representation, it 
should include a time value for the Last-Modified HTTP header. This is the last time 
the data underlying the representation was changed. For "a road map of the United 
States," the Last-Modified is likely to be the time the map imagery was first imported 
into the service. For "restaurants in New York City," the Last-Modified may only be a 
few days old: whenever a restaurant was last added to the database of places. For "con- 
tainer ships near San Francisco," the value of Last-Modified may be only a few minutes 
prior. 

The client can store this value of Last-Modified and use it later. Let's say the client 
requests "a road map of the United States" and gets a response that says: 

Last-Modified: Thu, 30 Nov 2006 20:00:51 GMT 

The second time the client makes a GET request for that resource, it can provide that 
time in the If-Modified-Since header: 



138 | Chapter 5: Designing Read-Only Resource-Oriented Services 



GET /road/Earth HTTP/l.l 
Host: maps.example.com 

If-Modified-Since: Thu, 30 Nov 2006 20:00:51 GMT 

If the underlying data changed between the two requests, the server sends a response 
code of 200 ("OK") and provides the new representation in the entity-body. That's the 
same thing that happens during a normal HTTP request. But if the underlying data has 
not changed, the server sends a response code of 304 ("Not Modified"), and omits any 
entity-body. Then the client knows it's okay to reuse its cached representation: the 
underlying data hasn't changed since the first request. 

There's a little more to it than that (again, I cover this in more detail in Chapter 8). But 
you can see the advantages. A client that fetches detailed maps is going to be making 
lots of HTTP requests. If most of those HTTP requests give a status code of 304, the 
client will be able to reuse old images and place lists instead of downloading new ones. 
Everyone saves time and bandwidth. 

What Might Go Wrong? 

I also need to plan for requests I can't fulfill. When I hit an error condition I'll send a 
response code in the 3xx, 4xx, or 5xx range, and I may provide supplementary data in 
HTTP headers. If they provide an entity-body, it'll be a document describing an error 
condition, not a representation of the requested resource (which, after all, couldn't be 
served) . 

I provide a full list of the HTTP response codes in Appendix B, along with examples 
where you might use each of them. Here are some likely error conditions for my map 
application: 

• The client may try to access a map that doesn't exist, like /road/Saturn. I under- 
stand what the client is asking for, but I don't have the data. The proper response 
code in this situation is 404 ("Not Found"). I don't need to send an entity-body 
along with this response code, though it's helpful for debugging. 

• The client may use a place name that doesn't exist in my database. The end user 
might have mistyped the name, or used a name the application doesn't recognize. 
They may have described the place instead of naming it, they might have the right 
name but the wrong planet. Or they might just be constructing URIs with random 
strings in them. 

I can return a 404 response code, as in the previous example, or I can try to be 
helpful. If I can't exactly match a requested place name, like /Earth/Mount%20Rush 
more%20National%20Monument, I might run it through my search engine and see if it 
comes up with a good match. If I do get a match, I can offer a redirect to that place: 
say, /Earth/43 -9; -95 . 9/Mount%20Rushmore. 

The response code for the helpful case here would be 303 ("See Other"), and the 
HTTP response header Location would contain the URI of the resource I think the 



The HTTP Response | 139 



client was "really" trying to request. It's the client's responsibility to take the hint 
and request that URI, or not. 

If I try a search and still have no idea what place the client is talking about, I'll 
return a response code of 404 ("Not Found"). 

• The client may use logically impossible latitudes or longitudes, like 500,-181 (500 
degrees north latitude, 181 degrees west longitude). A 404 ("Not Found") is a good 
response here, just as it is for a place that doesn't exist. But a 400 ("Bad Request") 
would be more precise. 

What's the difference between the two cases? Well, there's nothing obviously 
wrong with a request for a nonexistent place name like "Tanhoidfog." It just 
doesn't exist right now. Someone could name a town or a business "Tanhoidfog" 
and then it would be a valid place name. The client doesn't know there's no such 
place: one of the nice things a client can do with my map service is check to see 
which places really exist. 

But there is something wrong with a request for the latitude/longitude pair 
500,-181. The laws of geometry prevent such a place from ever existing. A mini- 
mally knowledgeable client could have figured that out before making the request. 
A 400 response code is appropriate in that case: the problem is the client's fault 
for even making the request. 

• A search for places on a map might return no search results. There might be no 
racing speedways near Sebastopol, CA. This is disappointing, but it's not an error. 
I can treat this like any other search: send a 200 response code ("OK") and a rep- 
resentation. The representation would include a link to the place that was searched, 
along with an empty list of search results. 

• The server may be overloaded with requests and unable to fulfil this particular 
request. The response code is 503 ("Service Unavailable"). An alternative is to 
refuse to handle the request at all. 

• The server may not be functioning correctly. This might be due to missing or cor- 
rupted data, a software bug, a hardware failure, or any of the other things that can 
go wrong with a computer program. In this case the response code is 500 ("Internal 
Server Error"). 

This a frustrating response code (the whole 5xx series is frustrating, actually) be- 
cause there's nothing the client can do about it. Many web application frameworks 
automatically send this error code when an exception happens on the server side. 

Conclusion 

I've now got a design for a map web service that's simple enough for a client to use 
without a lot of up-front investment, and useful enough to be the driver for any number 
of useful programs. It's so closely attuned to the philosophy of the Web that you can 



140 I Chapter 5: Designing Read-Only Resource-Oriented Services 



use it with a web browser. It's RESTful and resource-oriented. It's addressable, state- 
less, and well connected. 

It's also read-only. It assumes that my clients have nothing to offer but their insatiable 
appetites for my data. Lots of existing web services work this way, but read-only web 
services are only half the story. In the next chapter I'll show you how clients can use 
HTTP's uniform interface to create new resources of their own. 



Conclusion | 141 



CHAPTER 6 

Designing Read/Write Resource- 
Oriented Services 



In Chapter 5 I designed a fantasy web service that serves map images of various plan- 
ets, 'navigation information for moving around the map, and information about places 
on the planets: restaurants, meteor craters, and so on. That's a huge amount of data to 
serve, but it can all be contained in a premade data set. There's nothing a user can do 
to put his own data on the server. 

Clients for the map service in the previous chapter can do all sorts of interesting things 
with maps and places, but they can't rely on the server to track anything except the 
preset data. In this chapter I expand the scope of the map service. It becomes less like 
a search engine's web service and more like Amazon S3 and the Flickr and del.icio.us 
APIs. It not only serves data, it stores data on its clients' behalf. 

How open should I make the new service? A totally open service would allow users to 
provide their own versions of everything in the standard data set. Clients could create 
their own planets, and upload custom maps and databases of places. If I was too lazy 
to find map data myself (I am), I could even start with an empty database and allow 
the community to populate my entire data set. That's what del.icio.us and Flickr did. 

Is this a good idea? When designing a web service, which levers of state should you 
expose, and which should you keep to yourself? That depends on what your users want 
to do, and how much of their applications you're willing to write for them. 

A client uses a web service because the service has something it wants: some data, a 
place to store data, or a secret algorithm. A web service is an abstraction layer, like an 
operating system API or a programming language library. If you wrote a math library 
for working with infinite series, and all your users started using it to estimate the value 
of n, you'd probably add that feature as a higher-level library function. That way all 

Remember, I'm using "planets" as a shorthand for "bodies that can be addressed with longitude and latitude. " 
I don't just mean whatever 8 or 1 1 bodies the International Astronomical Union has decided are planets this 
week. 



143 



your users could use the same well-tested n-estimation code instead of each person 
writing his or her own implementation. Similarly, if all your users implement the same 
features on top of your web service, you might help them out by moving those features 
into the service. If all your users want to add certain kinds of custom data to the data 
set, you can start supporting a new kind of resource, so they don't have to define their 
own local structures. 

My goal here is fairly modest: to illustrate the concepts of resource-oriented service 
design. It's certainly possible to design a mapping service that starts off with an empty 
data set and gets everything through user contributions, but such a service would have 
more moving parts than I've got concepts to explain. If I decided to show you that 
service, this chapter would start out well, but once I'd explained all the concepts I'd 
still have a lot of domain-specific design work to do, and it would get boring. 

I want the map service to have about as many moving parts as I have new concepts to 
explain. I'm going to expand the previous chapter's service just enough so that clients 
can annotate the map with custom places. Every custom place is associated with a user 
account, and may be public or private to that account. 

User Accounts as Resources 

If I'm going to let anyone with a web service client annotate our worlds, I need some 
way of distinguishing his custom places from the standard places in my database. I'll 
also need a way to distinguish one user's places from everyone else's places. Basically, 
I need user accounts. 

When a client annotates Earth or Mars with a custom place, the place he has created 
is associated with his user account. This way a client can find his place later. If the client 
chooses to expose that place publicly, other clients will see links to it in the represen- 
tations they fetch. 

Most existing web services have some kind of system for letting people sign up for user 
accounts or "API keys." Even services that only give read-only access often make you 
sign up for an account, so they can track and ration your usage. If you've followed along 
with all of the examples in the book, by this time you have an Amazon Web Services 
account, a del.icio.us account, and a Flickr account. 

Yahoo! Web Services does things a little differently. Instead of tying the key to you 
personally, you can sign up for any number of application keys. You can distribute the 
application key with your application, and anyone can use it. Yahoo! tracks application 
usage, not individual usage. I registered the key "restbook" for a particular "applica- 
tion": this book. You and anyone else can use that key to run the sample Yahoo! Web 
Services code in this book. 

The procedure for signing up for these web accounts doesn't vary much. You use your 
web browser to go to a web site and fill out some HTML forms. You usually have to 



144 | Chapter 6: Designing Read/Write Resource-Oriented Services 



click through a legal agreement, and maybe respond to a verification email. Sometimes 
your web service account is tied to your preexisting account on the corresponding web 
site. 

The user account system I'm about to design works a little differently. In my map 
service, user accounts are resources, just like the maps themselves. In fact, they're my 
first read/write resources. My clients won't have to use their web browsers to sign up 
for a user account: they can create one with a generic web service client. 

Why Should User Accounts Be Resources? 

Why have I decided to design my user accounts differently from those of nearly every 
existing web service? I have two reasons. First: most web services make you sign up for 
an account through a web application. Web application design is a well-understood 
topic and it's not the topic of this book. Web services are indeed very similar to web 
applications, but resource creation is one of the places where they differ. The main 
difference here is that HTML forms currently support only GET and POST. This means 
web applications must use overloaded POST to convey any unsafe operation. If I tried 
to cover the typical method of getting a user account, I'd end up skimming the details 
as not relevant to web services. Treating user accounts as read/write resources means 
I can demonstrate the new resource-oriented design procedure on a data structure 
you're probably familiar with. 

Second, I want to show that new possibilities open up when you treat everyday data 
structures as resources, subject to the uniform interface. Consider an Internet-connec- 
ted GPS device that ties into my map service. Every hour or so, it annotates Earth (as 
exposed through my web service) with its current position, creating a record of where 
the GPS device is over time. 

There will be thousands of these devices, and each one should only be able to see its 
own annotations. The person in charge of programming the device should not be limi- 
ted to creating a single user account for personal use. Nor should everyone who buys 
the device have to go to my web site and fill out a form before they can use the device 
they bought. 

Since user accounts are resources, every one of these devices can have its own account 
on my web service (possibly with a username based on the serial number), and these 
accounts can be created automatically. They might be created in batches as the devices 
are manufactured, or each one may create an account for itself when its owner first 
turns it on. 

The end users may never know that they're using a web service, and they'll never have 
to sign up for a key. The device programmer does need to know how our web service 
works, and needs to write software that can create user accounts. If user accounts are 
resources, it's obvious how the device programmer can do this. HTTP's uniform in- 
terface gives most of the answers ahead of time. 



User Accounts as Resources | 145 



Authentication, Authorization, Privacy, and Trust 

Once I start exposing user accounts, I need some way of determining which user, if 
any, is responsible for a given HTTP request. Authentication is the problem of tying a 
request to a user. If you want to name a new place on Mars, I need some way of knowing 
that the new place should be associated with your user account instead of someone 
else's. Authorization is the problem of determining which requests to let through for a 
given user. There are some HTTP requests I'd accept from user A but reject from user 
B: requests like "DELETE user A" or "GET all of user A's private places." In my service, 
if you authenticate as user A, you're allowed to manipulate user A's account, but not 
anyone else's. 

I'll have more to say about RESTful modes of authentication and authorization in 
Chapter 8, but here are the basics. When a web service client makes an HTTP request, 
it may include some credentials in the HTTP header Authorization. The service exam- 
ines the credentials, and decides whether they correctly identify the client as a particular 
user (authentication), and whether that user is actually allowed to do what the client 
is trying to do (authorization). If both conditions are met, the server carries out the 
request. If the credentials are missing, invalid, or not good enough to provide author- 
ization, then the server sends a response code of 401 ("Unauthorized"). It sets the WWW- 
Authenticate response header with instructions about how to send correct credentials 
in the future. 

There are several standard kinds of authentication. The most common are HTTP Basic, 
HTTP Digest, and WSSE. Some web services implement custom forms of authentica- 
tion: in Chapter 3 I showed how Amazon S3 implements authentication with a so- 
phisticated request signing mechanism. It doesn't really matter which authentication 
mechanism I choose since I'm not actually implementing this service, but let's say I go 
with the simplest choice: HTTP Basic authentication. 

There's also the notion of privacy. Given that user A's list of private annotations can't 
be accessed by any other user, the representation of that list still needs to be transmitted 
over the Internet. The data's going to go through a lot of computers before it gets to 
the client. What's to stop one of those computers from examining the supposedly pri- 
vate list? To solve this problem I'm going to encrypt each HTTP transaction over SSL. 
In the previous chapter I presented URIs that started with http://maps.example.com/. 
In this chapter my URIs all start with https://maps.example.com/. 

Using HTTPS instead of HTTP prevents other computers from eavesdropping on the 
conversation between client and server. This is especially important when using HTTP 
Basic authentication, since that authentication mechanism involves the client sending 
its credentials in plain text. 

Now I've got a secure, trusted means of communication between the client and the 
server. But there's one more relationship to consider: the relationship between the client 
software and the human end user. Why should the end user trust the client software 
with its authentication credentials? Let me ask you a question to clarify the problem. 



146 | Chapter 6: Designing Read/Write Resource-Oriented Services 



Whenever you log in to a web site, you're trusting your web browser to send your 
username and password to that web site, and nowhere else. Why do you trust your 
browser with that information? How do you know your browser doesn't have a secret 
backdoor that broadcasts everything you type to some seedy IRC channel? 

There are several possible answers. You might be using an open source browser like 
Firefox, which has good source control and a lot of people looking at the source code. 
You might say there's safety in numbers: that millions of people use your brand of 
browser and there haven't been any problems traceable to the browser itself. You might 
monitor your network traffic to make sure your browser is only sending the data you 
tell it to send. But most people just take it on faith that their web browser is trustworthy. 

That's the human web. Now imagine I send you a cool new web service client for 
managing your del.icio.us bookmarks. Do you trust that client with your del.icio.us 
username and password? Do you trust it as much as you trust your web browser with 
the same information? Hopefully not! No web service client is as popular as a web 
browser, and no web service client has as many eyes on the source code. On the human 
web, we usually ignore the problem by taking a leap of faith and trusting our web 
browsers. On the programmable web the problem is more obvious. We don't neces- 
sarily trust our own clients with our authentication credentials. 

There's nothing in the HTTP standard to deal with this problem, because it's a problem 
between the end user and the client: HTTP lives between the client and the server. 
Solving this problem requires forgoing all the standard ways of sending authentication 
information: Basic, Digest, and WSSE don't work because they require the client to 
know the credentials. (You can solve it with Digest or WSSE by having a tiny, trusted 
account manager send encrypted authentication strings to the actual, untrusted client. 
I don't know of any web service clients that use this architecture.) 

Big names in web services like Google, Amazon, eBay, and Flickr have come up with 
ways for a client to make web service requests without knowing the actual authenti- 
cation credentials. You saw a hint of this in Chapter 3: 1 showed how to sign an Amazon 
S3 request and give a special URI to someone else, which they could use without 
knowing your password. I'll have more to say about this in Chapter 8. For now I just 
want you to know that there's a complication on the programmable web you might 
never have considered. Because there's not yet any standard way of solving this prob- 
lem, I'm going to punt on it for now and use HTTP Basic authentication for my services. 
My users will have to trust their clients as much as they trust their web browsers. 

Turning Requirements into Read/Write Resources 

Now that I've identified a new data set (user accounts), I'm going to go through the 
same design procedure I did for the data set I developed in the previous chapter (planets, 
places on the planets, maps of the planets, and points on the maps). But the procedure 
from the previous chapter only suffices for read-only resources. This chapter makes it 



User Accounts as Resources | 147 



possible for clients to create, modify, and delete resources. So I've added two steps to 
the procedure (steps 3 and 4) . 

1. Figure out the data set 

2. Split the data set into resources 
For each kind of resource: 

3. Name the resources with URIs 

4. Expose a subset of the uniform interface 

5. Design the representation (s) accepted from the client 

6. Design the representation (s) served to the client 

7. Integrate this resource into existing resources, using hypermedia links and forms 

8. Consider the typical course of events: what's supposed to happen? 

9. Consider error conditions: what might go wrong? 

Figure Out the Data Set 

Most sites with user accounts try to associate personal information with your account, 
like your name or email address. I don't care about any of that. In my map service, there 
are only two pieces of information associated with a user account: 

• The name of the account 

• A password used to access the account 

Each user account also has some subordinate resources (custom places on planets) 
associated with it, but I'll figure that part out later. All I need for now is a way of 
identifying specific user accounts (a username), and a way for a client to present cre- 
dentials that tie them to a certain user account (a password). 

Since I don't track any personal information, there's no reason apart from tradition to 
even call this a "user account. " I could call it a "password -protected set of annotations. " 
But I'll stick to the traditional terminology. This makes it easier to visualize the service, 
and easier for you to come up with your own enhancements to the user account system. 

Split the Data Set into Resources 

This was a fairly large step back in Chapter 5, when my data set was large and vague: 
"planets, places, and maps." Here the data set is fairly constrained: "user accounts." 
I'll expose each user account as a resource. In terms of the Chapter 5 terminology, these 
new resources are resources of the second type. They're the portals through which my 
service exposes its underlying user objects. Another site might also expose the list of 
user accounts itself as a one-off resource, or expose algorithmic resources that let a 
client search the list of users. I won't bother. 



148 | Chapter 6: Designing Read/Write Resource-Oriented Services 



Name the Resources with URIs 



This part is also easy, since I only have one kind of resource. I'll expose a user account 
with a URI of the following form: https://maps. example. com/ user / {user -name) . 

Expose a Subset of the Uniform Interface 

This is the first new step. I skipped it when designing read-only resources, because there 
was nothing to decide. By definition, read-only resources are the ones that expose no 
more than the HTTP methods GET, HEAD, and OPTIONS. Now that Pre designing 
resources that can be created and modified at runtime, I also have PUT, POST, and 
DELETE to consider. 

Even so, this step is pretty simple because the uniform interface is always the same. If 
you find yourself wishing there were more HTTP methods, the first thing to do is go 
back to step two, and try to split up your data set so you have more kinds of resources. 
Only if this fails should you consider introducing an element of the RPC style by making 
a particular resource support overloaded POST. 

To reiterate the example from Chapter 5: if you have resources for "readers," and re- 
sources for "published columns," and you start thinking "it sure would be nice if there 
was a SUBSCRIBE method in HTTP," the best thing to do is to create a new kind of 
resource: the "subscription." As HTTP resources, subscriptions are subject to HTTP's 
uniform interface. If you decide to forgo the uniform interface and handle subscriptions 
through overloaded POST on your "reader" resources, defining the interface for those 
resources becomes much more difficult. 

I can decide which bits of the uniform interface to expose by asking questions about 
intended usage: 

• Will clients be creating new resources of this type? Of course they will. There's no 
other way for users to get on the system. 

• When the client creates a new resource of this type, who's in charge of determining 
the new resource's URI? Is it the client or the server? The client is in charge, since 
the URI is made up entirely of constant strings (https://maps.example.com/user/) 
and variables under the client's control ({user-name}). 

From those two questions I get my first result. To create a user account, a client will 
send a PUT request to the account's URI. If the answer to the second question was "the 
server's in charge of the final URI," I'd expect my clients to create a user by sending a 
POST request to some "factory" or "parent" URI. See the "Custom Places" section later 
in this chapter for a case where the answer to the second question is "the server's in 
charge." 

• Will clients be modifying resources of this type? Yes. It's questionable whether or 
not a user should be allowed to change his username (I'm not going to allow it, for 
simplicity's sake), but a user should always be allowed to change his password. 



User Accounts as Resources | 149 



• Will clients be deleting resources of this type? Sure. You can delete an account when 
you're done with it. 

• Will clients be fetching representations of resources of this typePThis is up for debate. 
Right now there's not much information associated with a user account: only the 
username, which is part of the URI, and the password, which I won't be giving out. 

I'm going to say yes, which means I will be exposing GET and HEAD on user 
account resources. If nothing else, clients will want to see whether or not their 
desired username already exists. And once I allow users to define custom places, 
clients will want to look at the public places defined by specific users. 

Design the Representation(s) Accepted from the Client 

My data set comes with no built-in user accounts: every one is created by some client. 
The obvious next step in this design is to specify how the client is supposed to create 
a user account. 

Let's go back to Chapter 3 and Amazon S3 for a minute. A client creates an S3 bucket 
by sending an empty PUT request to the URI of the bucket. The client doesn't need to 
send an entity-body in the request, because the bucket has no state other than its name. 

To create an S3 object inside a bucket takes a little more work. An S3 object has two 
bits of state: name and value. The name goes into the URI, the destination of the PUT 
request. But the value needs to go into the entity-body of the PUT request. S3 will accept 
any data at all in this entity-body, because the whole point is that the value of an S3 
object can be anything, but there needs to be something there: you can't have an empty 
object. 

Most web services are a little pickier about what goes into the entity-body: it has to be 
in a certain format and convey certain bits of resource state. My user accounts have 
two elements of resource state: the username and the password. If a PUT request is 
going to succeed in creating a user account, it needs to convey both pieces of state. The 
username is included in the scoping information: any PUT request that creates an ac- 
count will have that account's username in the URI. What about the password? 

The client will send the new user's password in an entity-body, as part of a represen- 
tation. In Chapter 5, I introduced representations as documents the server sends the 
client: a way for the server to convey the state of a resource. Representations flow the 
other way, too. They're how a client suggests changes to the state of a resource. When 
you PUT an S3 object, the entity-body you send is a representation of the object. The 
representation you send with a PUT request is an assertion about the new state of a 
resource. 

In "Representing the List of Planets" in Chapter 5 I considered several possible repre- 
sentation formats. I looked at plain text, JSON, XML using a made-up vocabulary, and 
Atom (XML again, but using a preexisting vocabulary). I decided on XHTML, a pre- 
existing XML vocabulary oriented around marking up human-readable documents. In 



150 | Chapter 6: Designing Read/Write Resource-Oriented Services 



that chapter the question was what format would be most useful when served to the 
client. Now, the question is how the client should format its proposed state changes. 
What format makes it easiest for the client to convey a password to the server? 

When the state is complex, it's helpful for the server to accept the same representation 
format it sends. The client can request a representation with GET, modify the repre- 
sentation, and then PUT it back, committing its changes to the underlying resource 
state. As we'll see in Chapter 9, the Atom Publishing Protocol uses this technique ef- 
fectively. And, of course, S3 serves the representation of an object byte for byte the way 
it was when the client first PUT it into the system. S3 doesn't even pretend to know 
anything about the meaning of the representations it serves. 

Here, I've only got one item of state (the password), and it's not one that the server will 
ever send to the client. Now's a good time to introduce a representation format for 
simple cases like these. 



Form-encoding 

This representation doesn't have an official name beyond its media type (application/ 
x-www-form-urlencoded), but you've probably seen it before. It's sometimes called "CGI 
escaping." When you submit an HTML form in your web browser, this is the format 
the browser uses to marshal the form data into something that can go into an HTTP 
request. Consider an HTML form like the one in Example 6-1. 

Example 6-1. A simple HTML form 

<form action="http: //www. example.com/color-pairs" met hod=" POST" > 

<input name="colorl" type="text"/> 

<input name="color2" type="text"/> 
</form> 

If the user enters the values "blue" and "green" in the text fields colorl and color2 
fields, a form-encoded representation of that data would be 
colorl=blue&color2=green. When the form is submitted, the browser makes a POST 
request to http://www.example.com/color-pairs, and sends colorl=blue&color2=green 
in the entity-body: that's a representation. If the form's "method" attribute were GET, 
then when the user submitted the form the browser would make a GET request to 
http://www.example.com/color-pairs?colorl=blue&color2=green. That's got the same 
data in the same format, but there the data is scoping information that identifies a 
resource, not a representation. 

When an object's state can be represented as key-value pairs, form-encoding is the 
simplest representation format. Almost every programming language has built-in fa- 
cilities for doing form-encoding and -unencoding: they're usually located in the lan- 
guage's HTTP or CGI library. 



My map service accepts a form-encoded representation when a client tries to create or 
edit a user. The only pieces of state I've associated with a user are its name and 



User Accounts as Resources | 151 



password. The name goes into the URI and I've decided it can't change, so my user 
representations just look like "password={ the-password}" . Example 6-2 is hypothetical 
Ruby code for creating a user account with the map service. 

Example 6-2. Hypothetical map client to create a user account 
require 'rubygems' 
require 'rest-open-uri' 
require 'cgi' 
require 'uri' 

def make_user(username, password) 
open("https: //maps. example. com/user/#{URI.escape(username)}", 

:data => CGI: :escape("password=#{password}"), :method => :put) 

end 



A couple things to note here. First, I've started transmitting sensitive 
data (passwords) over the network, so I'm now using HTTPS. Second, 
fl-T; I'm actually using two different kinds of encoding in this code sample. 
The username, which goes into the URI, is URI-encoded using 
URI. escape. The password, which goes into the representation, is form- 
encoded with CGI: : escape. URI-encoding is similar to form-encoding, 
but it's not the same, and confusing them is a common source of subtle 
bugs. 



Changing an account's password is the same as creating the account in the first place. 
The client sends a PUT request to the account URI, with a new representation of the 
account (that is, the new password). Of course, no one can change an account's pass- 
word without authorization. To modify a user account, a client must also provide an 
Authorization header that convinces my service it has the right to modify that account. 
In short, changing a user's password requires knowing the current password. As I said 
earlier, my service expects incoming Authorization headers to conform to the HTTP 
Basic authentication standard. 

A DELETE request never requires a representation, but deleting a user from my service 
will require a proper Authorization header. That is: to delete a user account you must 
know that user's password. 



Design the Representation(s) to Be Served to the Client 

A client will GET a user account's URI to retrieve a representation of a user account, 
just as a client GETs the URI of a map or a place to retrieve a representation of that 
map or place. What should the representation of a user account look like? 

Right now it won't look like much, since I've only got two pieces of state to convey, 
and one of them (the password) I don't want to be sending out. Indeed, in a well- 
designed system I won't even have the password to send out. I'll only have an encrypted 
version of it, for use in authentication. Once I integrate custom places into this repre- 
sentation, it'll look better. For now, Example 6-3 is a fairly sparse XHTML document. 



152 | Chapter 6: Designing Read/Write Resource-Oriented Services 



Example 6-3. A representation of "your" user's account 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 

<head> 

<title>User homepage for leonardr</title> 
</head> 
<body> 

<p class="authenticated"> 
You are currently logged in as 

<a class="user" href="/user/leonardr">leonardr</a>. 
</p> 

<p>User homepage for 

<a class="user" href="/user/leonardr">leonardr</ax/p> 

<form id="modifyUser" method="put" action=""> 
<p>Change your password: 

<input class="password" name="password" /xbr /> 

<input class="submit" /></p> 
</form> 

</body> 
</html> 

Once again I'm using the representation to convey the current resource state, and to 
help the client drive to other states. I used an HTML form to describe a future PUT 
request the client might make if it wants to change the user's password (an item of 
resource state). Note that there's no form telling the client how to get a representation, 
or how to delete this user. It's taken for granted that you use HTTP GET and DELETE 
for that. I only need hypermedia for complicated things: links to other resources (so 
the client knows which URI to GET or DELETE), and descriptions of representations. 



f /; As with the 



You may have noticed a problem in Example 6-3. Its form specifies an 
HTTP method of PUT, but HTML forms only allow GET and POST. 

repeat" syntax in Example 5-11, I'm using the as-yet-un- 
released XHTML 5 to get around the shortcomings of the current ver- 
sion of HTML. Another way to handle this is to send a WADL snippet 
instead of an HTML form, or use the trick described in Chapter 8 to run 
PUT requests over overloaded POST. 



If you GET someone else's user account, you'll be served a different representation, 
similar to the one in Example 6-4. 

Example 6-4. A representation of someone else's user account 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 



User Accounts as Resources | 153 



<head> 

<title>User homepage for samruby</title> 
</head> 
<body> 

<p class="authenticated"> 
You are currently logged in as 

<a class="user" href="/user/leonardr">leonardr</a>. 
</p> 

<p>User homepage for <a class="user" href="/user/samruby">samruby</ax/p> 

</body> 
</html> 

This representation has no controls for altering the state of the resource, because the 
client isn't authorized to do that: the client authenticated as leonardr and this is sam 
ruby's page. Right now the representation does nothing but confirm to leonardr that a 
user named samruby exists. If there was no such user, a GET request to /user/samruby 
would give a status code of 404 ("Not Found"), and the client would be free to create 
samruby with a PUT request. 

Link This Resource to Existing Resources 

In the previous chapter I defined several classes of resource: the list of maps, individual 
maps, places, and lists of places (that is, lists of search results) . None of these are directly 
relevant to user accounts, but there are a couple of nice features I can add at this point. 

One nice feature is to add the "authenticated" message (seen in the two sample repre- 
sentations above) to the representation of every resource. It'll be displayed whenever 
the client submits a request with valid credentials. The "authenticated" message is a 
piece of hypermedia that shows an authenticated client how to retrieve data about its 
user account. Every resource is now connected to the user account of the user who 
requested it. 

Another nice piece of hypermedia would be one that shows an unauthenticated client 
how to create a user account. The best place for this bit of hypermedia would be the 
representation of the list of planets: after all, that's the service's "homepage." It already 
contains links to the other main parts of the service, so it should contain a link to this 
new part. 

Once again, HTML hypermedia isn't quite up to the job. And once again, I'm going to 
use XHTML 5, which makes minor changes to HTML, rather than introduce a totally 
new technology like WADL in the middle of a chapter. Example 6-5 is an XHTML 5 
snippet that tells a client how to create a user. 



154 | Chapter 6: Designing Read/Write Resource-Oriented Services 



Example 6-5. Hypermedia describing how to create a user account 

<form id="createUser" method="PUT" template="/user/{username}"> 
<p>Username: <input type="text" name="username" /xbr /> 
<p>Password: <input type="password" name="password" /xbr /> 
<input class="submit" /> 
</form> 




The two deviations from the HTML you're familiar with are in the 
method attribute (like Example 6-3, it specifies PUT where HTML 4 al- 
£ lows only GET and POST), and the brand-new template attribute, 
which inserts a form variable ("username") into the URI using the URI 
Templating standard (http://www.ietf.org/internet-drafts/draft-gregorio- 
uritemplate-OO.txt) . 

As of the time of writing, URI Templating was a proposed addition to 
HTML 5, but it hadn't been approved. It's possible that it will be rejec- 
ted, and that Example 6-5 won't be valid HTML 5 any more than it is 
valid HTML 4. In that case you can use URI Templating unofficially 
(forcing your users to write custom clients), or switch to WADL. 



The hypermedia form talks about the syntax of the PUT request, but it can't say much 
about the semantics. A web service client can read the HTML form in Example 6-5, 
but its understanding is limited. It knows that the form is labelled "createUser" but it 
doesn't know what "createUser" means. It knows that if it PUTs a certain representa- 
tion to a certain URI, the server will probably accept it. It knows what PUT means, 
because PUT always means the same thing. It knows that the representation should 
include a "username," but it doesn't know a username from an ostrich. It takes a human 
being — a programmer — to understand what a user is, that "createUser" means "create 
a user," what a username is, and all the rest. A programmer needs to set the rules about 
when and how user accounts are created. This piece of hypermedia does nothing but 
tell the client how to structure the PUT request when it comes time to "createUser," 
whatever that means. It's a promise from the web service to the client. 

Many web services put all of this data up front, in a single WSDL or WADL file, for 
the ease of the client programmer. This is somewhat contrary to the REST design phi- 
losophy because it violates, or at the very least subverts, the principle of connectedness. 
But in web services, where the client must be programmed in advance, it's an under- 
standable impulse, and often it doesn't cause any problems. 

What's Supposed to Happen? 

Let's consider what might happen when a client sends a PUT request to /user/leo- 
nardr. As is usual with HTTP, the server reads this request, takes some action behind 
the scenes, and serves a response. I need to decide which numeric response code the 
response will have, and what HTTP headers and/or entity-body will be provided. I also 



User Accounts as Resources | 155 



need to decide how the request will affect resource state: that is, what real-world effects 
it will have. 

It's not hard to see what happens if all goes well with a PUT request. If there's no user 
called "leonardr," the service creates one with the specified password. The response 
code is 201 ("Created"), and the Location header contains theURI of the newly created 
user. 

If the user account already exists, the resource state is modified to bring it in line with 
the client's proposed new representation. That is, the account's password is modified. 
In this case the response code may be 200 ("OK"), and the response entity-body may 
contain a representation of the user account. Or, since the password change never 
affects the representation, the response code may be 205 ("Reset Content") and the 
response entity-body may be omitted altogether. 

PUT requests are the only complicated ones, because they're the only ones that include 
a representation. GET and DELETE requests work exactly according to the uniform 
interface. A successful GET request has a response code of 200 ("OK") and a repre- 
sentation in the entity-body. A successful DELETE request also has a response code of 
200 ("OK"). The server can send an entity-body in response to a successful DELETE, 
but it would probably contain just a status message: there's no longer a resource to 
send a representation of. 

What Might Go Wrong? 

A request that creates, modifies, or deletes a resource has more failure conditions than 
one that just retrieves a representation. Here are a few of the error conditions for this 
new resource. 

The most obvious problem is that the client's representation might be unintelligible to 
the server. My server expects a representation in form-encoded format; the client might 
send an XML document instead. The status code in this case is 415 ("Unsupported 
Media Type"). 

Alternatively, the client might not have provided a representation at all. Or it might 
have provided a form-encoded representation that's ill-formed or full of nonsense data. 
The status code in this case is 400 ("Bad Request"). 

Maybe the representation makes sense but it tells the server to put the resource into an 
inconsistent or impossible state. Perhaps the representation is "password=", and I don't 
allow accounts with empty passwords. The exact status code depends on the error; in 
the case of the empty password it would probably be 400 ("Bad Request"). In another 
situation it might be 409 ("Conflict"). 

Maybe the client sends the wrong credentials, sends authorization credentials for a 
totally different user account, or doesn't send the Authorization header at all. A client 
can only modify or delete a user if it provides that user's credentials. The response code 



156 | Chapter 6: Designing Read/Write Resource-Oriented Services 



in this case is 401 ("Unauthorized"), and I'll set the WWW-Authenticate header with 
instructions to the client, giving a clue about how to format the Authorization header 
according to the rules of HTTP Basic authentication. 

If the client tries to create a user that already exists, one possible response code is 409 
("Conflict"). This is appropriate because carrying out the PUT request would put the 
service's resources into an inconsistent state: there'd be two user resources with the 
same username. Another possibility is to treat the PUT request as an attempt to change 
an existing user's password without providing any authentication, and send a response 
code of 401 ("Unauthorized"). 

As in the previous chapter, there might be an unspecified problem on the server side: 
response code 500 ("Internal Server Error") or 503 ("Service Unavailable"). 

Custom Places 

Now I'm ready to go through the resource design procedure all over again. This time 
I'm designing the custom places clients can create: places that will show up on maps 
alongside the built-in places. Hopefully you're getting the hang of the procedure by 
now (if not, take heart: I'll do it some more in the next chapter), so this trip through it 
will be somewhat abbreviated. This time I want to focus on what makes custom places 
different from user accounts. 

Figure Out the Data Set 

A web service client can create any number of places on any of the planets for which I 
have maps. Custom places will show up in lists of search results, just like the built-in 
places from the previous chapter. Custom places can have the same data as built-in 
places: a type ("city"), a name ("Springfield"), coordinates of latitude and longitude 
("39.81E89.64W"), and a textual description ("The capital of Illinois"). Many custom 
places may share the same coordinates ("My house" and "My current location"), and 
a custom place may share a location with a built-in place. 

Every custom place is associated with some user account. Custom places may be public 
or private. A private place is visible and modifiable only to someone who provides the 
credentials for the user account that "owns" the place. 

Split the Data Set into Resources 

Each custom place will be a resource, just as every built-in place is. I also want to let 
clients get a list of their custom places. In my design, a user account is just a password- 
protected list of places, so I won't be exposing the place list as a separate resource. 
Instead I'll expand the "user account" resource so it encompasses a user's list of places. 
This is analogous to the way a bucket in Amazon S3 is represented as nothing but a list 
of objects. 



Custom Places | 157 



Name the Resources with URIs 

A custom place is clearly a subordinate resource, but subordinate to what? I could 
reasonably associate it with a user account, a geographic point on some planet, or an 
enclosing place like a city, country, or planet. Which of these relationships should I 
capture with my URIs? 

I've chosen to name custom places much the same way I name built-in places. Each 
place is associated with a geographic point, and can be accessed with a URI of the 
form /usei/{username}/ {planet}/ {latitude} , {longitude} / {place name}. The new el- 
ement is {usemame}, intended to distinguish between different people's views of the 
same place: for instance, Sam's review of Joe's Diner at /user/samruby /Earth/ 
45.2;-114.2/Joe's%20Diner and Leonard's less glowing review at /user/leonardr/Earth/ 
45.2;-114.2/Joe's%20Diner. 

A URI like /Earth/USA?show=Doe's+Diner works like it did before: it returns search re- 
sults for places called "Joe's Diner," anywhere in the U.S. The only difference is that 
now there are more possible places to search: not only the built-in database of places, 
but each user's public list of places, and your own private list. 

Built-in places are still privileged. As it happens, there's a Japanese theme park that 
includes a one-third scale model of Mount Rushmore. If a client creates a custom place 
called "Mount Rushmore" north of Tokyo, /Earth/Hount%20Rushmore still points to the 
original in South Dakota. It doesn't suddenly become ambiguous which "Mount Rush- 
more" resource that URI refers to. However, /Earth?show=Mount+Rushmore will show 
both places. 

Expose a Subset of the Uniform Interface 

Clients can use GET and HEAD to retrieve representations of built-in places, their own 
places (whether public or private), and public places created by others. Clients can 
delete their own places with DELETE, and change the state of their places with PUT. 

There are two ways a client might create a map annotation. The client might add a 
comment to an existing place on the map ("Mount Rushmore"), or it might give a new 
name to a certain point of latitude and longitude ("the cornfield where I kissed Betty"). 

In the first case, the resource being created is "Mount Rushmore (from leonardr's point 
of view) . " When creating this resource the client shouldn't have to know exactly where 
on the map Mount Rushmore is. "Mount Rushmore" is a consensus name and there's 
a built-in place by that name. The client can rely on the server to look up the coordi- 
nates. In the second case, the resource being created is a brand new place that the 
server's never heard of, and the client is responsible for knowing the coordinates. 

How can I work this feature into my resource-oriented design? "Mount Rushmore 
(from leonardr's point of view)" is a subordinate resource of another resource: the built- 
in place "Mount Rushmore." This resource already exists and has a URI: one of them 



158 | Chapter 6: Designing Read/Write Resource-Oriented Services 



is /Earth/Mount%20Rushmore. If the client wants to reuse the consensus name for a place, 
it shouldn't have to look up its location. Instead of figuring out the final URI of the 
annotation and sending a PUT request to it, the client can send a POST request to the 
"Mount Rushmore" URI and let the server figure out the ultimate URI. 

Similarly, if the client wants to comment on the Alabama capitol building, it can POST 
to /Earth/USA/AL/State%20capitol instead of figuring out the exact coordinates or street 
address. Any URI that identifies a built-in place can be the target of a POST request 
that comments on that place. 

What about custom names? What if a client wants to give the name "Mount Rushmore" 
not to the original in South Dakota, but to the scale model in Imaichi? What if the client 
wants to create an annotation for "the cornfield where I kissed Betty"? 

Here the client must know the latitude and longitude of the place it wants to create. 
This means it'll have all the information necessary to create the URI of the new resource: 
the world, a geographic point on the world, the name of the place, and its own user- 
name. The client could make a PUT request to a URI like /user/bob/Earth/42;-93.7/ 

the%20cornf ield%20where This would work just like creating a user account by 

sending a PUT request to /user/bob. 

Even here, it's cleaner to use POST. A brand-new place on the map is a subordinate 
resource: it's subordinate to some point on the planet, just like a comment on a built- 
in place is subordinate to a place on the planet. So a client could also put a new place 
on the map by sending a POST request to /Earth/42; -93 -7. It works the same way as 
a comment on existing places (a POST to /Earth/Hount%20Rushmore), except here the 
place is identified by latitude and longitude, not by consensus name. 

My service will support POST for brand-new places because that's simpler. The inter- 
face will be the same whether you're adding a brand new place to the planet, or making 
a comment on some consensus place. Another service might support both methods: 
PUT to the final URI if the client is willing to figure out that URI, and POST to a parent 
URI if it's not. 

Finally, note that although I'm using POST, it's not overloaded POST. Clients of my 
service use POST only when they want to create a resource "beneath" an existing one. 
The URI of the new resource (/user/leonardr/Earth/43-9; -103.46/Mount%20Rushmore) 
may not directly extend the URI of the old (/Earth/Mount%20Rushmore), but the resources 
have a conceptual relationship. 

Design the Representation(s) Accepted from the Client 

When the client sticks a pin into a planet and creates a custom place, what information 
does it need to provide? It must identify a planet and a place on that planet: the spot 
where the pin goes. The place can be identified either by latitude and longitude, or by 
reference to a canonical name like "Mount Rushmore." Call these variables planet, 
latitude, longitude, and name. The server must know what type of place the client is 



Custom Places | 159 



putting on the map. A place may be public or not, and the client may provide a custom 
description of the place. The final URI also incorporates a username, but the client is 
already providing that, in the Authorization header. There's no need to make the client 
send that information twice. 

These are all key-value pairs. I can have clients represent places the way they represent 
user accounts: as form-encoded strings. There are no complex data structures here that 
might call for a JSON or XML representation. 

Client requests may choose to send some key-value pairs and omit others. Information 
that's in the URI as scoping information doesn't need to be repeated in the represen- 
tation. When the client sends a POST to /Earth/43 -9; -103 .46 it doesn't need to specify 
latitude and longitude, because that information's in the URI. It does need to specify 
name and type. 

When the client sends a POST to /Earth/Mount%20Rushmore it shouldn't specify 
latitude, longitude, or name. The client is making a new place based on a well-known 
existing place, and the new place will inherit the name and location of the existing 
place. The client may specify a custom type ("national-park," "political," "places in 
North Dakota") or inherit the default ("monument"). 

The client may always choose to omit description and public. My service sets default 
values for those variables: descriptions are empty by default, and places are public by 
default. 

When the client modifies one of its custom places, anything and everything about the 
place might change: its name, its location, its type, its description, or its public status. 
The PUT request that modifies a place can specify the same key-value pairs used to 
create a place, in any combination. The server will make the appropriate changes, as- 
suming the changes make sense. 

Example 6-6 shows a sample HTTP POST request that creates a new custom place. 
Combined, the form-encoded representation and the scoping information in the URI 
convey all required states for the new resource. The name and location of the new 
resource come from the scoping information; its type and description come from the 
representation. Since the representation doesn't specify a value for public, the default 
takes over and this new resource is made public. 

Example 6-6. An HTTP request that creates a subordinate resource 

POST /Earth/USA/Mount%20Rushmore HTTP/l.l 
Host: maps.example.com 

Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ= 
type=national-park&description=We%20visited%20on%203/5/2005 



160 | Chapter 6: Designing Read/Write Resource-Oriented Services 



Design the Representation(s) Served to the Client 

Most of the work here is already done. In Chapter 5 I defined an XHTML-based rep- 
resentation format for places. Custom places look the same as places from the built-in 
database. 

The only new part is this: when an authenticated client requests a representation of 
one of its custom places, our service will tack onto the representation some hypermedia 
showing the client how to edit that place (see Example 6-7). I don't need to tell clients 
how to delete the place: the uniform interface takes care of that. But I do need to convey 
the information I wrote in prose above: that a place is defined by planet, latitude, 
longitude, and so on. 

Example 6-7. A hypermedia form showing the client how to edit one of its places 

<form id="modifyPlace" method="PUT" action=""> 
<p>Modify this place:</p> 

<P> 

Name: <input name="name" value="Mount Rushmore" type="text" /xbr /> 
Type: <input name="type" value="national-park" type="text" /xbr /> 
Position: 

<input name="latitude" value="43.9" type="text" />, 
<input name="longitude" value="-103.46" type="text" /xbr /> 
Description: 

<textarea name="description">We visited on 3/5/2005</textareaxbr /> 
Public? 

<input name="public" type="checkbox" value="on"/> 
<input type="submit" /> 
</p> 
</form> 

The caveats from earlier apply here too. This isn't valid XHTML 4, though it is valid 
XHTML 5, because it specifies PUT as its method. Also, a client doesn't know what to 
do with this form unless it's been programmed in advance. Computers don't know 
what "modifyPlace" means or what data might be a good value for "latitude." 

Because clients have to be programmed in advance to understand these forms, most of 
today's services don't include a form for modifying a resource in that resource's rep- 
resentation. They either serve all the forms up front (in a WSDL or WADL file), or they 
specify them in prose (as I did above) and leave it for the service programmer to figure 
out. It's debatable whether it's really helpful to serve forms along with representations, 
but serving them is better than just specifying the API in prose and making the pro- 
grammer implement it. 

Link This Resource to Existing Resources 

I've got three kinds of integration to do. The first is data integration. When you DE- 
LETE a user account, the account's custom places — everything under /user/fuser 



Custom Places | 161 



name} — should also be deleted. URIs to these resources used to work, but now they will 
return a response code of 410 ("Gone") or 404 ("Not Found"). 

The other kinds of integration should be familiar by now. They involve changing the 
representations of existing resources to talk about the new one. I want search results 
to link to custom places. I want points on the globe to show how the user can create a 
custom place at that point. I want to improve my connectedness by connecting "custom 
place" resources to the resources I defined already. 

The rather empty-looking representation of a user's account, seen in Example 6-3, 
badly needs some link-based integration. This is the ideal place to list a user's custom 
places. I'll represent the place list with the same XHTML list of links I use to represent 
search results. 

In the service defined in Chapter 5, a client that searched for places called "Mount 
Rushmore" (/Earth?show=Mount+Rushmore) would only find places from my built-in 
place database: probably only the "consensus" location of Mount Rushmore in South 
Dakota. In the new version of the service, there's likely to be more than one result. In 
the new version, that search will also return other users' annotations for Mount Rush- 
more, and other places that users have named "Mount Rushmore," like the scale model 
in Imaichi. 

This is the same case as in Chapter 5, where the built-in place database contained more 
than one "Joe's diner." I present search results in a list, each linking to a specific re- 
source. All I'm doing is expanding the search. A search result may be a place in the 
built-in database, a custom place created by some other user and exposed publicly, or 
a custom place created by the authenticated user (which may be public or private). 

I also need to show the client how to create its own places on the map. Custom places 
are created as subordinate resources of existing places. The logical thing to do is to put 
that information in the representations of those places: places with URIs like /Earth/ 
Mount%20Rushmore and /Earth/42; -93. 7. 

Example 6-8 is a possible representation of /Earth/43. 9;-103. 46 that brings together 
most of what I've covered in the past two chapters. This representation abounds in 
hypermedia. It links to a certain point on several different maps, a place from the built- 
in database, custom places from other users, and a custom place created by the au- 
thenticated user. It also has a hypermedia form that will let the authenticated user create 
a new custom place at these coordinates. Compare this representation to the smaller 
representation of /Earth/43- 9; -103.46 back in Example 5-9. 

Example 6-8. An XHTML representation of 43.9N 103.46W on Earth 

<!D0CTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 

" http : / /www . w3 . org/TR/xhtmlll/DTD/xhtmlll . dtd " > 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 

<head> 

<title>43.9°N 103.46Sdeg;W on Earth</title> 
</head> 



162 I Chapter 6: Designing Read/Write Resource-Oriented Services 



<body> 

<p class="authenticated"> 
You are currently logged in as 

<a class="user" href="/user/leonardr">leonardr</a>. 
</p> 

<P> 

Welcome to 

<a class="coordinates" href="/Earth/43.9,-l03.46">43.9°N 
103.46°W</a> 
on scenic <a class="place" href="/Earth">Earth</a>. 
</p> 

<p>See this location on a map:</p> 

<ul class="maps"> 
<lixa class="map" href="/road/Earth/43.9;-l03.46">Road</ax/li> 
<lixa class="map" href="/satellite/Earth/43.9;-l03.46">Satellite</ax/li> 

</ul> 

<p>Places at this location:</p> 

<ul class="places"> 
<li> 

<a class="builtin" href="Mount%20Rushmore">Mount Rushmore</a> 
System data says: 

<span class="description">The world's largest sculpture</span> 
</li> 

<li> 

<a class="custom" href="Mt.%20Rushmore/userl">Mt. Rushmore</a> 
<a class="user" href="/users/userl">userl</a> says: 

<span class="description">Built on land stolen from the Lakota tribe</span> 
</li> 

<li> 

<a class="custom" href="Mount%20Rushmore%20Gift%20Shop/user2"> 
Mount Rushmore Gift Shop 
</a> 

<a class="user" href="/users/userl">userl</a> says: 
<span class="description">Best fudge I've ever had</span> 
</li> 

<li> 

<a class="custom-private" href="Mount%20Rushmore/leonardr">Mt. Rushmore</a> 
You said: <span class="description">We visited on 3/5/2005</span> 
</li> 
</ul> 

<form id="searchPlace" method="get" action=""> 
<p> 

Show nearby places, features, or businesses: 

<input name="show" repeat="template" /> <input class="submit" /> 



Custom Places | 163 



</p> 

</form> 

<form id="createPlace" method="post" action=""> 
<p>Create a new place here:</p> 

<p> 

Name: <input name="name" value="" type="text" /xbr /> 
Type: <input name="type" value="" type="text" /xbr /> 
Description: 
<textarea name="description"x/textareaxbr /> 
Public? 

<input name="public" type="checkbox" value="on"/> 
<input type="submit" /> 
</p> 
</form> 

</body> 
</html> 

What's Supposed to Happen? 

This new resource, the custom place, mostly works like other resources I've already 
defined. A custom place responds to GET just like a built-in place. It responds to PUT 
(with a representation consisting of key-value pairs) and DELETE (with no represen- 
tation) just like "user account" resources do. I only have a couple new edge cases to 
consider here. 

When the client creates a custom place, the response code is 201 ("Created"). This 
works the same way as users. But it was never possible to cause a user's URI to change, 
because I prohibited users from changing their usernames. It's possible to change the 
name of a place, or to move one (say, a ship) from one point on the map to another. 
Either of these actions will change the URI. 

When the client modifies a custom place without changing its location, the response 
code will be 200 ("OK") . If the location changes, the response code will be 301 ("Moved 
Permanently") and the Location header will contain the place's new URI. The client is 
responsible for updating its data structures to keep track of the new URI. This ties into 
a debate I'll revisit in Chapter 8, about whether it's more important to have URIs that 
contain useful information, or URIs that never change. My URIs describe a custom 
place using two pieces of resource state: coordinates and name (/ user/leonardr/Earth/ 
43.9;-103.46/Mt.%20Rushmore). If either of those changes, the old URI breaks. 

Broken URIs are no fun on the human web, and they're even less fun on the program- 
mable web. If my custom "place" is a ship or something else that's constantly moving, 
it effectively has no permanent URI. This is the single biggest design flaw in my system. 
If I were exposing this as a real web service, I'd probably give a "permalink" to every 
place: an alternate URI that doesn't incorporate any changeable resource state. Since 
everything about a place can change except the planet it's on and the person who owns 



164 | Chapter 6: Designing Read/Write Resource-Oriented Services 



it, these URIs will not look very friendly: my annotation of Mount Rushmore might be 
accessible from /user/leonardr/Earth/36028efa8. But at least they'll always refer to the 
same place. 

What Might Go Wrong? 

This new kind of resource introduces new error conditions, but most of them are var- 
iations of ones I've already covered, so I'll pass over them quickly. The client might try 
to move an existing place off of the map by providing an invalid latitude or longitude: 
the response code is 400 ("Bad Request"), just as it was in a similar case in Chapter 5. 
The 400 response code is also appropriate when a client tries to create a place without 
providing all the information the server needs. This is similar to the 400 response code 
the server sends if the client tells the server to change a user's password, but doesn't 
actually provide the new password. 

My service doesn't allow a single user to define more than one place with the same 
name at the same coordinates. /user/leonardr/Earth/43.9;-103.46/Mt.%20Rushmore 
can only identify one place at a time. Suppose a client has two places called "My car," 
and makes a PUT request that would move one to the location of the other. My service 
rejects this request with a response code of 409 ("Conflict"). There's nothing wrong 
with moving a place to a certain set of coordinates; it's just that right now there happens 
to be another place with that name there. The same 409 response code would happen 
if the client had two custom places at the same coordinates, and tried to rename one 
to match the name of the other. In either case, the client is making a syntactically valid 
request that would put the system's resources into an inconsistent state. It's the same 
as trying to create a user that already exists. 

There's one totally new error condition worthy of attention: the client may try to access 
a private place created by someone else. There are two possibilities. The first is to deny 
access with response code 403 ("Forbidden"). The 403 response code is used when the 
client provides no authentication, or insufficient authentication; the latter certainly 
applies in this case. 

But a response code of 403 is a tacit admission that the resource exists. The server 
should not be giving out this information. If client A creates a custom place and marks 
it private, client B should not be able to figure out anything about it, even its name, 
even by guessing. When revealing the existence of a resource would compromise se- 
curity, the HTTP standard allows the server to lie, and send a response code of 404 
("Not Found"). 

A Look Back at the Map Service 

This is still a simple design but it's got quite a few features. In Chapter 5 my clients 
could get map images, navigate around a map, bookmark points on the globe, and do 
geographic searches against a built-in database of places. Now they can keep track of 



A Look Back at the Map Service | 165 



custom places, register comments on consensus places, and share places with other 
users. The representation in Example 5-6 shows off most of these features. 

All of these features are made available through resources that expose the uniform 
interface. Occasionally I need to supplement the uniform interface with hypermedia 
forms (here, the XHTML 5 forms) that tell the client what representations can go with 
a PUT or POST request. The vast majority of requests will be GET requests. These need 
no hypermedia supplements, because GET always means the same thing. 

A client can get right to its desired resource by constructing a URI, or it can get to that 
resource by navigating links in the hypermedia I serve. You can get anywhere from the 
service root (the list of planets) by following links and filling out forms. Each resource 
is fairly simple, but the service as a whole is very powerful. The power comes from the 
variety of resources, the links that connect them, and the fact that each resource is 
individually addressable. 

The Resource-Oriented Architecture sets down a list of design questions you need to 
ask yourself. I embodied these questions in the previous chapter's seven-step design 
procedure, and this chapter's extended nine-step procedure. Like any architecture, the 
ROA imposes design constraints, but it doesn't make all the design decisions for you. 
There are many other ways to define a map service in a RESTful and resource-oriented 
way. It all depends on how you split the data set into resources, what representations 
you define for those resources, and how you tie them together with hypermedia. 

What I've designed should work and be useful to clients, but I won't know for sure, 
because I don't have to implement it. I just designed it to illustrate concepts in a book. 
When designing a real service, you also have implementation issues to consider. You 
have to write code to back up every decision you make: decisions about what resources 
you expose, what parts of the uniform interface they respond to, what URIs you choose, 
and which representations you serve and accept. In the next chapter, I'll make all these 
decisions again for a different data set, and this time I'll back it up with a real 
implementation. 



166 | Chapter 6: Designing Read/Write Resource-Oriented Services 



CHAPTER 7 



A Service Implementation 



It's been a while since I presented any code. Indeed, coming up with the code is currently 
a general problem for REST advocates. Despite the simplicity of REST, right now there 
are few well-known services that show off its principles. The average web service has 
an architecture that combines elements of REST with the RPC style. This is changing, 
of course, and this book is part of the wave of change. Another problem is that many 
services seem trivial when exposed through resources, even though they'd look very 
impressive as SOAP/WSDL services. See Appendix A for a partial list of real RESTful 
services with enough moving parts to learn from. 

Until recently, web frameworks made few concessions to the lucrative REST market. 
They focus on applications for web browsers, using only the GET and POST methods 
of HTTP. You can implement RESTful services with just GET and POST, but the lim- 
itation seems to encourage the RPC style instead. New frameworks for RESTful services 
are showing up, though, and existing frameworks are changing to accommodate REST 
fans. Django (Python), Restlet Qava), and Ruby on Rails all make it easy to expose 
resources that respond to HTTP's uniform interface. I cover these frameworks in 
Chapter 12. In this chapter I use Ruby on Rails as a medium for demonstrating how to 
implement a real-world web service. 

A Social Bookmarking Web Service 

Back in Chapter 2 I introduced del.icio.us, a web site that lets you publicly post book- 
marks, tag them with short metadata strings, and see which URIs other people have 
posted. There's also a del.icio.us web service, which I used as the target of the web 
service clients in Chapter 2. 

I mentioned that the del.icio.us web service has a couple shortcomings. First, it's a 
REST-RPC hybrid, not a fully RESTful service. It only exposes resources by accident 
and it doesn't respect HTTP's uniform interface. Second, the web service only gives 
you access to your own bookmarks and tags. When you use the service, it looks like 
you're the only person on del.icio.us. In this chapter I use Ruby on Rails to develop a 



167 



RESTful web service has much of the functionality of the del.icio.us web service and 
the del.icio.us web site. 

I've got three goals for this chapter. Previous chapters showed you service design from 
first principles. Here, I want to show you how to make a RESTful, resource-oriented 
service out of an existing RPC-style service. Second, I want to show you the sort of 
tradeoffs you might need to make to get a design that works within your chosen frame- 
work. Finally, I want to show you the complete code to a nontrivial web service, without 
boring you with page after page of implementation details. I chose Ruby on Rails as my 
framework because Ruby is a dynamic language, and Rails comes with a lot of helper 
classes. This makes it easy to illustrate the underlying concepts in just a few lines of 
code. What's more, the most recent version of Rails is explicitly designed around the 
principles of REST and resource-oriented design. 

My challenge is to reconcile the constraints imposed by the Resource-Oriented Archi- 
tecture, and my own design sensibilities, with the simplifying assumptions of the Rails 
framework. My resource design is heavily informed by what Rails itself considers good 
design, but at points I've had to hack Rails to get the behavior I want, instead of the 
behavior Rails creator David Heinemeier Hansson wants. Rails imposes more con- 
straints than most frameworks (this is a big reason for its success, actually), but your 
choice of framework will always have some effect on your design. 

I'm going to start with an empty Rails 1 .2 application, and fill in the details as the design 
takes shape. I created a Rails application with the following command: 
$ rails bookmarks 

I installed two Rails plugins I know I'll need: acts_as_taggable, to implement tags on 
bookmarks, and http_authentication, to tie HTTP Basic authentication into my user 
model. I've also installed the atom-tools Ruby gem, so I can generate Atom feeds for 
representations. 

$ cd bookmarks 

$ script/plugin install acts_as_taggable 
$ script/plugin install http_authentication 
$ gem install atom-tools 

I also created a SQL database called bookmarks_development, and configured config/ 
database.yaml so that Rails can connect to the database. 

Figuring Out the Data Set 

Because I'm basing my service on an existing one, it's fairly easy to figure out the pa- 
rameters of the data set. If what follows is confusing, feel free to flip back to "del.icio.us: 
The Sample Application" in Chapter 2 for an overview of del.icio.us. 

The del.icio.us site has four main kinds of data: user accounts, bookmarks (del.icio.us 
calls them "posts"), tags (short strings that act as metadata for bookmarks), and bun- 



168 | Chapter 7: A Service Implementation 



dies (collections of tags for a user). The web site and the web service track the same 
data set. 

Unlike an S3 bucket, or a user account on my map service, a del.icio.us user account 
is not just a named list of subordinate resources. It's got state of its own. A del.icio.us 
account has a username and password, but it's supposed to correspond to a particular 
person, and it also tracks that person's full name and email address. A user account 
also has a list of subordinate resources: the user's bookmarks. All this state can be 
fetched and manipulated through HTTP. 

A bookmark belongs to a user and has six pieces of state: a URI, a short and a long 
description, a timestamp, a collection of tags, and a flag that says whether or not it's 
public (the previous chapter's "custom place" resource has a similar flag). The client 
is in charge of specifying all of this information for each bookmark, though the URI 
and the short description are the only required pieces of state. 

The URIs in users' bookmarks are the most interesting part of the data set. When you 
put a bunch of peoples' bookmarks together, you find that the URIs have emergent 
properties. On del.icio.us these properties include newness, a measure of how recently 
someone bookmarked a particular URI; "popularity," a measure of how many people 
have bookmarked that URI; and the "tag cloud," a generated vocabulary for the URI, 
based on which tags people tend to use to describe the URI. The del.icio.us web site 
also exposes a recommendation engine that relates URIs to each other, using a secret 
algorithm. 

I'm not going to do much with the emergent properties of URIs, properties that account 
for much of del.icio.us's behind-the-scenes code. My implemented service will have a 
notion of newness but it won't have popularity, tag clouds, or recommendation algo- 
rithms. This is just so I can keep this book down to a manageable size instead of turning 
it into a book about recommendation algorithms. 

Tags have only one piece of state: their name. They only exist in relation to bookmarks 
— and bundles, which I haven't described yet. A bundle is a user's decision to group 
particular tags together. A user with tags "recipes," "restaurants," and "food," might 
group those tags into a bundle called "gustation." I'll show the RESTful design of bun- 
dles, just for completeness, but I won't be implementing them when it comes time to 
write code. 

At this point I know enough about the data set to create the database schema. I create 
an empty database called bookmarks_development in my MySQL installation, and put 
this data in the file db/migrate/001_initial_schema.rb, shown in Example 7-1. 

Example 7-1. The bookmark database schema as a Rails migration 
class InitialSchema < ActiveRecord: :Migration 

# Create the database tables on a Rails migration, 
def self. up 

# The 'users' table, tracking four items of state 



Figuring Out the Data Set | 169 



# plus a unique ID. 

create_table :users, :force => true do |t| 

t. column :name, :string 

t. column :full_name, : string 

t. column :email, :string 

t. column :password, :string 
end 

# The 'bookmarks' table, tracking six items of state, 

# plus a derivative field and a unique ID. 
create_table :bookmarks, :force => true do |t| 

t. column :user_id, :string 
t. column :uri, : string 

t. column :uri_hash, :string # A hash of the URI. 

# See book text below, 
t. column :short_description, :string 
t. column :long_description, :text 
t. column :timestamp, :datetime 
t. column :public, :boolean 
end 

# This join table reflects the fact that bookmarks are subordinate 

# resources to users. 

create_table :user_bookmarks, :force => true do |t| 

t. column :user_id, :integer 

t. column :bookmark_id, :integer 
end 

# These two are standard tables defined by the acts_as_taggable 

# plugin, of which more later. This one defines tags. 
create_table :tags do |t| 

t. column :name, :string 
end 

# This one defines the relationship between tags and the things 

# tagged--in this case, bookmarks. 
create_table :taggings do |t| 

t. column :tag_id, :integer 
t. column :taggable_id, :integer 
t. column :taggable_type, :string 
end 

# Four indexes that capture the ways I plan to search the 

# database. 

add_index : users, :name 
add_index : bookmarks, :uri_hash 
add_index :tags, :name 

add_index :taggings, [:tag_id, :taggable_id, :taggable_type] 
end 



170 | Chapter 7: A Service Implementation 



# Drop the database tables on a Rails reverse migration, 
def self. down 

[:users, :bookmarks, :tags, :user_bookmarks, :taggings] .each do |t| 

drop_table t 
end 
end 
end 

I've used Ruby code to describe five database tables and four indexes. I create the 
corresponding database schema by running this command: 

$ rake dbrmigrate 

Resource Design 

In Chapters 5 and 6 1 had a lot of leeway in turning my imaginary data set into resources . 
The idea for my map service came from the Google Maps application with its image 
tiles, but I took it off in another direction. I added user accounts, custom places, and 
other features not found in any existing map service. 

This chapter works differently. I'm focusing on translating the ideas of del.icio.us into 
the Resource-Oriented Architecture. There are lots of ways of exposing a data set of 
tagged bookmarks, but I'm focusing on the ones del.icio.us actually uses. Let's start by 
taking a look at what the del.icio.us web service has to offer. 

The del.icio.us web service is a REST-RPC hybrid service, described in English prose 
at http://del.icio.us/help/api/. The web service itself is rooted at https://api.del.icio.us/ 
vll '. The service exposes three RPC-style APIs, rooted at the relative URIs posts/, 
tags/, and bundles/. Beneath these URIs the web service exposes a total of twelve RPC 
functions that can be invoked through HTTP GET. I need to define RESTful resources 
that can expose at least the functionality of these three APIs: 

First, the posts/ API, which lets the user fetch and manage her bookmark posts to 
del.icio.us: 

• posts/get: Search your posts by tag or date, or search for a specific bookmarked 
URL 

• posts/recent: Fetch the n most recent posts by the authenticated user. The client 
may apply a tag filter: "fetch the n most recent posts that the authenticated user 
tagged with tag t". 

• posts/dates: Fetch the number of posts by the authenticated user for each day: 
perhaps five posts on the 12th, two on the 15th, and so on. The client may apply 
a tag filter here, too. 

• posts/all: Fetch all posts for the authenticated user, ever. The client may apply a 
tag filter. 

• posts/ update: Check when the authenticated user last posted a bookmark. Clients 
are supposed to check this before deciding to call the expensive posts/all. 



Resource Design | 171 



• posts/add: Create a bookmark for a URL The client must specify a short descrip- 
tion. It may choose to specify a long description, a set of tags, and a timestamp. A 
bookmark may be public or private (the default is public). A client may not book- 
mark the same URI more than once: calling posts/ add again overwrites the old post 
with new information. 

• posts/delete: Deletes a user's post for a particular URI. 

Second, the tags/ API, which lets the authenticated user manage her tags separately 
from the bookmarks that use the tags: 

• tags/ get: Fetch a list of tags used by the authenticated user. 

• tags/rename: Rename one of the authenticated user's tags. All posts tagged with 
the old name will now be tagged with the new name instead. 

Finally, the bundles API, which lets the authenticated user group similar tags together. 

• tags/bundles/all: Fetch the user's bundles. The resulting document lists the bun- 
dles, and each bundle lists the tags it contains. 

• tags/bundles/set: Group several tags together into a (possibly new) bundle. 

• tags/bundles/delete: Delete a bundle. 

That's the web service. As I mentioned in Chapter 2, the service only gives you access 
to your own bookmarks and tags. The del.icio.us web site has social features as well, 
and I'm going to steal some of those features for my design. 

Here are some interesting "functions" exposed by the del.icio.us web site but not the 
web service: 

• /{username}: Fetch any user's bookmarks. 

• / '{username} / '{tag}: Fetch any user's bookmarks, applying a tag filter. 

• /tag/ '{tag- name}: Fetch bookmarks tagged with a particular tag, from all users. 

• /url/ {URI-MD5}: Fetch the list of users who have bookmarked a particular URI. The 
{URI-MD5} happens to be the MD5 hash of the URI, but from the average client's 
point of view that's not important: it's an opaque string of bytes that somehow 
identifies a URI within the del.icio.us system. 

• /recent: Fetch the most recently posted bookmarks, from all users. The del.icio.us 
home page also shows this information. 

Now that I know what the service has to do, arranging the features into resources is 
like working a logic puzzle. I want to expose as few kinds of resources as possible. But 
one kind of resource can only convey one concept, so sometimes I need to split a single 
feature across two kinds of resource. On the other hand, sometimes I can combine 
multiple RPC functions into one kind of resource, a resource that responds to several 
methods of HTTP's uniform interface. 



172 | Chapter 7: A Service Implementation 



REST in Rails 



I'm not designing these resources in a vacuum: I'm going to implement them in a Rails 
application. It's worth taking a brief look at how RESTful applications work in Rails. 
Unlike some other frameworks, Rails doesn't let you define your resources directly. 
Instead, it divides up an application's functionality into controllers: it's the controllers 
that expose the resources. The first path variable in a request URI is used to route Rails 
to the appropriate controller class. For instance, in the URI /weblogs/4 the "weblogs" 
designates the controller: probably a class called WeblogController. The "4" designates 
the database ID of a particular weblog. 

In previous versions of Rails, programmers defined RPC-style methods on controllers: 
methods like rename and delete. To rename a weblog you'd send a GET or an over- 
loaded POST request to /weblogs/4/rename. Rails applications, like most web applica- 
tions, were REST-RPC hybrids. 

In Rails 1.2, programmers define special controller methods that correspond to the 
methods of HTTP's uniform interface. For instance, sending a GET to /weblogs triggers 
the WeblogController's index method, which is supposed to retrieve a list of the we- 
blogs. Sending a POST to the same URI triggers the WeblogControllerttcreate method, 
which creates a subordinate resource beneath /weblogs: say, a weblog with a URI 
of /weblogs/4. The Rails controller exposes a resource — "the list of weblogs" — that 
responds to GET and POST. As you'd expect, when you POST to the "list" resource 
you get a subordinate resource: a new weblog. 

The subordinate resource also supports the uniform interface. If you wanted to rename 
a weblog in an RPC-style service, you might POST a new name to /weblogs/4/rename. 
Under a RESTful regime, you PUT a new name to /weblogs/4, triggering the 
WeblogController#update method. To delete a weblog, you send a DELETE request to 
its URI, triggering the controller's WeblogController#destroy method. There's no need 
to expose an RPC-style URI /weblogs/4/delete, because HTTP's uniform interface al- 
ready knows about deleting. 

These two resources, a list and an item in the list, show up all the time. Every database 
table is a list that contains items. Anything that can be represented as an RSS or Atom 
feed is a list that contains items. Rails defines a RESTful architecture that makes a 
simplifying assumption: every resource you expose can be made to fit one of these two 
patterns. This makes things easy most of the time, but the cost is aggravation when you 
try to use Rails controllers to expose resources that don't fit this simple model. 

I'm going to define my resources in terms of Rails controllers. These controllers impose 
constraints on my URI structure and my use of the uniform interface, and I need to 
design with those constraints in mind. By the time I'm done designing the controllers, 
I'll know which resources the controllers expose, which URIs they answer to, and which 
methods of the uniform interface correspond to which RPC functions from the 
del.icio.us service. Basically, I'll have completed steps 2 through 4 of the 9-step proce- 
dure from the "Turning Requirements into Read/Write Resources" section in 



Resource Design | 173 



Chapter 6: "Split the data set into resources," "Name the resources with URIs," and 
"Expose a subset of the uniform interface." In Chapter 12 I give a variant of the service 
design procedure specifically for Rails services. 

I'll only be accessing my Rails application from my local machine. The root URI will 
be http://localhost:3000/vl . When I give a relative URI below, like /users, understand 
that I'm talking about http://localhost-.3000/vl/users. I only ever plan to write one ver- 
sion of this service, but I'm versioning the URIs, just in case. (When and how to version 
is discussed in Chapter 8). 

The User Controller 

Now I'm going to go back to that big list of RPC functions I found in the del.icio.us 
API and web site, and try to tease some Rails controllers out of it. One obvious controller 
is one that exposes information about user accounts. In Rails, this would be a class 
called UsersController. As soon as I say that, a lot of decisions are made for me. Rails 
sets up a path of least resistance that looks like this: 

The user controller exposes a one-off "user list" resource, at the URI /users. It also 
exposes a resource for every user on the system, at a URI that incorporates the user's 
database ID: /users/52 and the like. These resources expose some subset of HTTP's 
uniform interface. Which subset? Rails defines this with a programming-language in- 
terface in the superclass of all controller classes: ActionController: :Base. Table 7-1 
shows how the two interfaces line up. 

Table 7-1. How Rails wants my UsersController to look 



Operation 


HTTP action 


Rails method 


List the users 


GET /users 


UsersControlleritindex 


Create a user 


POST /users 


UsersController#create 


View a user 


GET /users/52 


UsersController#show 


Modify a user 


PUT /users/52 


UsersControlleritupdate 


Delete a user 


DELETE /users/52 


UsersController#destroy 



So if I want to let clients create new user accounts, I implement 
UsersController#create, and my "user list" resource starts calling that method in re- 
sponse to POST requests. 

The path of least resistance is pretty good but I have a couple problems with it. First, 
I don't want to let clients fetch the list of users, because del.icio.us doesn't have that 
feature. (Presumably the del.icio.us administrative interface does have a feature like 
this.) That's fine: I don't have to expose GET on every resource, and I don't have to 
define index in every controller. My user list resource, at the URI /users, will only 
expose the POST method, for creating new users. My user list is a featureless container 
for user account resources, and the only thing a client can do with it is create a new 



174 | Chapter 7: A Service Implementation 



account. This incorporates functionality like that at https://secure.del.icio.us/register, 
where you can use your web browser to sign up for a del.icio.us account. 



User Account Creation 

The real del.icio.us site doesn't expose user account creation through its web service. 
To create a user account you must prove you're a human being, by typing in the string 
you see in a graphic. This graphic (called a CAPTCHA) is an explicit attempt to move 
the human web off of the programmable web, to prevent automated clients (many of 
which are up to no good) from creating their own del.icio.us accounts. 

This is legitimate. Not every piece of functionality has to be part of your web service, 
and it's your decision what to expose. But I don't want to get into the details of web 
site and CAPTCHA design in this book, so I'm exposing user account creation as part 
of the web service. 



Another problem is that URIs like /users/52 look ugly. They certainly don't look like 
http://del.icio.us/leonardr, the URI to my corresponding page on del.icio.us. This URI 
format is the Rails default because every object in a Rails application's database can be 
uniquely identified by its table ("users") and its ID ("52"). This URI might go away (if 
user 52 DELETEs her account), but it will never change, because database unique IDs 
don't change. 

I'd rather expose readable URIs that might change occasionally than permanent URIs 
that don't say anything, so I'm going to identify a user using elements of its resource 
state. I happen to know that users have unique names, so I'm going to expose my "user" 
resources at URIs like /users/leonardr. Each resource of this type will expose the 
methods GET, PUT, and DELETE. This incorporates the functionality of the del.icio.us 
web site's / { useiname) "function. " It also incorporates the pages on the web site (I didn't 
mention these earlier) that let you edit and delete your own del.icio.us account. 

To expose this RESTful interface, I just need to implement four special methods on 
UsersController. The create method implements POST on the "user list" resource 
at /users. The other three methods implement HTTP methods on the "user" resources 
at /users/{username}: show implements GET, update implements PUT, and destroy 
implements DELETE. 

The Bookmarks Controller 

Each user account has a number of subordinate resources associated with it: the user's 
bookmarks. I'm going to expose these resources through a second controller class, 
rooted beneath the "user account" resource. 

The base URI of this controller will be /users/{username}/bookmarks. Like the users 
controller, the bookmarks controller exposes two types of resource: a one-off resource 
for the list of a user's bookmarks, and one resource for each individual bookmark. 



Resource Design | 175 



Rails wants to expose an individual bookmark under the URI /users/{username}/book 
marks/{database-id}. I don't like this any more than I like /users/{database-id}. I'd 
like the URI to a bookmark to have some visible relationship to the URI that got 
bookmarked. 

My original plan was to incorporate the target URI in the URI to the bookmark. That 
way if I bookmarked http://www.oreilly.com/, the bookmark resource would be avail- 
able at /vl/ users/leonardr/bookmarks/http : //www. oreilly . com/. Lots of services work 
this way, including the W3C's HTML validator (http://validator.w3.org/) . Looking at 
one of these URIs you can easily tell who bookmarked what. Rails didn't like this URI 
format, though, and after trying some hacks I decided to get back on Rails's path of 
least resistance. Instead of embedding external URIs in my resource URIs, I'm going to 
put the URI through a one-way hash function and embed the hashed string instead. 

If you go to http://del.icio.us/url/55020a5384313579a5flle75cl818b89 in your web 
browser, you'll see the list of people who've bookmarked http://www.oreilly.com/. 
There's no obvious connection between the URI and its MD5 hash, but if you know 
the former you can calculate the latter. It's certainly better than a totally opaque data- 
base ID. And since it's a single alphanumeric string, Rails handles it with ease. My 
bookmark resources will have URIs like /vl/users/leonardr/bookmarks/ 
55020a5384313579a5f Ile75cl8l8b89. That URI identifies the time I bookmarked http:// 
www.oreilly.com/ (see Example 7-2). 

Example 7-2. Calculating an MD5 hash in Ruby 
require 'digest/mds' 

Digest : : MD5 . new( " http : / /www . oreilly . com/" ) . to_s 
# => "55020a5384313579a5flle75cl8l8b89" 

When a user is first created it has no bookmarks. A client creates bookmarks by sending 
a POST request to its own "bookmark list" resource, just as it might create a user 
account by sending a POST to the "user list" resource. This takes care of the 
posts/add and posts/delete functions from the del.icio.us API. 



Creating a New Bookmark 

There are two other ways to expose the ability to create a new bookmark. Both are 
RESTful, but neither is on the Rails path of least resistance. 

The first alternative is the one I chose for user accounts back in Chapter 6. In the fantasy 
map application, a client creates a user account by sending a PUT request to /users/ 
{username}. The corresponding solution for the user bookmark would be to have a 
client create a bookmark by sending a PUT request to /users/{username}/bookmarks/ 
{URI-MD5}. The client knows its own username and the URI it wants to bookmark, and 
it knows how to calculate MD5 hashes, so why not let it make the final URI itself? 

This would work fine within the ROA, but it's not idiomatic for Rails. The simplest 
way to create new objects in RESTful Rails is to send a POST request to the corre- 
sponding "list" resource. 



176 | Chapter 7: A Service Implementation 



The other alternative treats bookmarks as a subordinate resource of user accounts. To 
create a bookmark you send a POST request, not to /users/{username}/bookmarks but 
to /users/{username}. The bookmark is made available at /users/{username}/{URI- 
MD5}. The "bookmarks" path fragment doesn't exist at all. 

Those URIs are more compact, but Rails doesn't support them (at least not very easily), 
because it needs that extra path fragment /bookmarks to identify the 
BookmarksController . There's also no easy way of exposing POST on an individual user. 
The method UsersController#create, which responds to POST, is already being used 
to expose POST on the user list. 

It's not a big deal in this case, but you can see how a framework can impose restrictions 
on the resource design, atop the rules and best practices of the Resource-Oriented 
Architecture. 



Unlike with the list of users, I do want to let clients fetch the list of a user's bookmarks. 
This means /users/{username}/bookmarks will respond to GET. The individual book- 
marks will respond to GET, PUT, and DELETE. This means the BookmarksControl 
ler: index, create, show, update, and delete. 

The "bookmark list" resource incorporates some of the functionality from the 
del.icio.us API functions posts/get, posts/recent, and posts/all. 

The User Tags Controller 

Bookmarks aren't the only type of resource that conceptually fits "beneath" a user 
account. There's also the user's tag vocabulary. I'm not talking about tags in general 
here: I'm asking questions about which tags a particular user likes to use. These ques- 
tions are handled by the user tags controller. 

This controller is rooted at /users/{wsernome}/tags. That's the "user tag list" resource. 
It's an algorithmic resource, generated from the tags a user uses to talk about her book- 
marks. This resource corresponds roughly to the del.icio.us tags/get function. It's a 
read-only resource: a user can't modify her vocabulary directly, only by changing the 
way she uses tags in bookmarks. 

The resources at /useis/{username}/tags/{tag} talk about the user's use of a specific 
tag. My representation will show which bookmarks a user has filed under a particular 
tag. This class of resource corresponds to the / '{username} / '{tag} "function" from the 
web site. It also incorporates some stuff of the del.icio.us API functions posts/get, 
posts/recent, and posts/all. 

The "tag" resources are also algorithmic, but they're not strictly read-only. A user can't 
delete a tag except by removing it from all of her bookmarks, but I do want to let users 
rename tags. (Tag deletion is a plausible feature, but I'm not implementing it because, 
again, del.icio.us doesn't have it.) So each user-tag resource will expose PUT for clients 
who want to rename that tag. 



Resource Design | 177 



Instead of PUT, I could have used overloaded POST to define a one-off "rename" 
method like the del.icio.us API's tag/rename. I didn't, because that's RPC-style think- 
ing. The PUT method suffices to convey any state change, whether it's a rename or 
something else. There's a subtle difference between renaming the tag and changing its 
state so the name is different, but it's the difference between an RPC-style interface and 
a uniform, RESTful one. It's less work to program a computer to understand a generic 
"change the state" than to program it to understand "rename a tag." 

The Calendar Controller 

A user's posting history — her calendar — is handled by one more controller that lives 
"underneath" a user account resource. The posting history is another algorithmically 
generated, read-only resource: you can't change your posting history except by posting. 
The controller's root URI is /users/{wserno/ne//calendar, and it corresponds to the 
del.icio.us API's posts/dates function. 

I'll also expose a variety of subresources, one for each tag in a user's vocabulary. These 
resources will give a user's posting history when only one tag is considered. These 
resources correspond to the del.icio.us API's posts/dates function with a tag filter ap- 
plied. Both kinds of resource, posting history and filtered posting history, will expose 
only GET. 

The URI Controller 

I mentioned earlier that URIs in a social bookmarking system have emergent properties. 
The URI controller gives access to some of those properties. It's rooted at /uris/, and 
it exposes URIs as resources independent from the users who bookmark them. 

I'm not exposing this controller's root URI as a resource, though I could. The logical 
thing to put there would be a huge list of all URIs known to the application. But again, 
the site I'm taking for my model doesn't have any feature like that. Instead, I'm exposing 
a series of resources at /uiis/{URI-MD5}: one resource for each URI known to the ap- 
plication. The URI format is the same as /users/{username}/bookmarks/{URI-MD5} in 
the user bookmark controller: calculate the MD5 hash of the target URI and stick it 
onto the end of the controller's base URI. 

These resources expose the application's knowledge about a specific URI, such as 
which users have bookmarked it. This corresponds to the /uil/{URI-MD5} "function" 
on the del.icio.us web site. 

The Recent Bookmarks Controller 

My last implemented controller reveals another emergent property of the URIs. In this 
case the property is newness: which URIs were most recently posted. 



178 | Chapter 7: A Service Implementation 



This controller is rooted at /recent. The top-level "list" resource lists all the recently 
posted bookmarks. This corresponds to the /recent "function" on the del.icio.us web 
site. 

The sub-resources at /iecent/{tag} expose the list of recently posted bookmarks that 
were tagged with a particular tag. For instance, a client can GET /recent/recipes to 
find recently posted URIs that were tagged with "recipes". This corresponds to 
the /tag/ {tag-name} function on the del.icio.us web site. 

The Bundles Controller 

Again, I'm not going to implement this controller, but I want to design it so you can 
see I'm not cheating. This controller is rooted at /user/fusernamej/bundles/. An alter- 
native is / user/ { username }/ tags/bundles/, but that would prevent any user from having 
a tag named "bundles". A client can send a GET request to the appropriate URI to get 
any user's "bundle list" . A client can POST to its own bundle list to create a new bundle. 
This takes care of tags/bundles/all and part of tags/bundles/set. 

The sub-resources at /user/ '{username} /bundles /{bundle} expose the individual bun- 
dles by name. These respond to GET (to see which tags are in a particular bundle), PUT 
(to modify the tags associated with a bundle), and DELETE (to delete a bundle). This 
takes care of tags/bundles/delete and the rest of tags/bundles/set. 

The Leftovers 

What's left? I've covered almost all the functionality of the original del.icio.us API, but 
I haven't placed the posts/ update function. This function is designed to let a client avoid 
calling posts/all when there's no new data there. Why bother? Because the 
posts/all function is extremely expensive on the server side. A del.icio.us client is 
supposed to keep track of the last time it called posts/all, and check that time against 
the "return value" of posts/update before calling the expensive function again. 

There's already a solution for this built into HTTP: conditional GET. I cover it briefly 
in "Conditional HTTP GET" later in this chapter and I'll cover it in more detail in 
"Conditional GET," but in this chapter you'll see it implemented. By implementing 
conditional GET, I can give the time- and bandwidth-saving benefits of posts/update 
to most of the resources I'm exposing, not just the single most expensive one. 

Remodeling the REST Way 

I've taken an RPC-style web service that was only RESTful in certain places and by 
accident, and turned it into a set of fully RESTful resources. I'd like to take a break now 
and illustrate how the two services line up with each other. Tables 7-2 through 7-6 
show every social bookmarking operation I implemented, the HTTP request you'd send 



Resource Design | 179 



to invoke that operation on my RESTful web service, and how you'd invoke the cor- 
responding operation on del.icio.us itself. 

Table 7-2. Service comparison: user accounts 



Operation 


On my service 




Create a user account 


POST /users 


POST /register (via web site) 


View a user account 


GET/users/{username} 


GET /users/{username} (via web site) 


Modify a user account 


PUT /users/{username} 


Various, via web site 


Delete a user account 


DELETE /users/{username} 


POST /settings/{username}/prof ile/delete (via 
web site) 



Table 7-3. Service comparison: bookmark management 



Post a bookmark 


POST/users/{username}/bookmarks 


GET /posts/add 


Fetch a bookmark 


GET/users/{username}/bookmarks/{URI-MD5} 


GET /posts/get 


Modify a bookmark 


PUT/users/{username}/bookmarks/{URI-MD5} 


GET /posts/add 


Delete a bookmark 


DELETE /users/{username}/bookmarks/{URI-MD5} 


GET /posts/delete 


See when the user last 


Use conditional HTTP GET 


GET /posts/update 


posted a bookmark 






Fetch a user's posting 


GET /users/{username}/calendar 


GET /posts/dates (your his- 


history 




tory only) 


Fetch a user's posting 


GET /users/{username}/calendar/ {tag} 


GET /posts/dates with 


history, filtered by tag 




query string (your history only) 


Table 7-4. Service comparison: finding bookmarks 


Fetch a user's recent 


GET /users/{username}/bookmarks with query 


GET /posts/recent (your 


bookmarks 


string 


bookmarks only) 


Fetch all of a user's 


GET/posts/{username}/bookmarks 


GET /posts/all (your book- 


bookmarks 




marks only) 


Search a user's bookmarks 


GET /posts/{username}/bookmarks with query 


GET /posts/get with query 


by date 


string 


string (your bookmarks only) 


Fetch a user's bookmarks 


GET/posts/{username}/bookmarks/{tag} 


GET /posts/get with query 


tagged with a certain tag 




string (your bookmarks only) 



Table 7-5. Service comparison: social features 



See recently posted bookmarks GET /recent GET /recent (via web site) 

See recently posted bookmarks for a certain tag GET /recent/{tag} GET /tag/{tag} (via web site) 



180 | Chapter 7: A Service Implementation 



See which users have bookmarked a certain URI GET / uris/{URI-MD5} GET /url/{URI-MD5} (via web site) 



Table 7-6. Service comparison: tags and tag bundles 



Operation 

Fetchauser'stagvocabu- 
lary 

Rename a tag 

Fetch the list of a user's 
tag bundles 

Group tags into a bundle 



On my service 

GET /users/{username}/tags 



PUT/users/{username}/tags/{tag} GET /tags/rename 

GET /users/{username}/bundles GET /tags/bundles/all (your 



POST / users/{username}/bundles GET /tags/bundles/set 

GET /users/{username}/bundles/{bundle} N/A 

PUT / users/{username}/bundles/{bundle} GET /tags/bundles/ set 

DELETE /users/{username}/bundles/{bundle} GET /tags/bundles/delete 



On del.icio.us 



GET /tags/get (your tags only) 



bundles only) 



Fetch a bundle 



Modify a bundle 



Delete a bundle 



I think you'll agree that the RESTful service is more self-consistent, even accounting 
for the fact that some of the del.icio.us features come from the web service and some 
from the web site. Table 7-6 is probably the best for a straight-up comparison. There 
you can distinctly see the main advantage of my RESTful service: its use of the HTTP 
method to remove the operation name from the URI. This lets the URI identify an object 
in the object-oriented sense. By varying the HTTP method you can perform different 
operations on the object. Instead of having to understand some number of arbitrarily- 
named functions, you can understand a single class (in the object-oriented sense) whose 
instances expose a standardized interface. 

My service also lifts various restrictions found in the del.icio.us web service. Most no- 
tably, you can see other peoples' public bookmarks. Now, sometimes restrictions are 
the accidental consequences of bad design, but sometimes they exist for a reason. If I 
were deploying this service commercially it might turn out that I want to add those 
limits back in. I might not want user A to have unlimited access to user B's bookmark 
list. I don't have to change my design to add these limits. I just have to change the 
authorization component of my service. I make it so that authenticating as userA doesn't 
authorize you to fetch userFJ's public bookmarks, any more than it authorizes you to 
delete userB's account. Or if bandwidth is the problem, I might limit how often any 
user can perform certain operations. I haven't changed my resources at all: I've just 
added additional rules about when operations on those resources will succeed. 



Ready for some more code? I've split my data set into Rails controllers, and each Rails 
controller has divided its data set further into one or two kinds of resources. Rails has 
also made decisions about what my URIs will look like. I vetoed some of these decisions 



Implementation: The routes.rb File 



Resource Design | 181 



(like /users/52, which I changed to /users/leonardr), but most of them I'm going to 
let stand. 

I'll implement the controllers as Ruby classes, but what about the URIs? I need some 
way of mapping path fragments like bookmarks/ to controller classes like 
BookmarksController. In a Rails application, this is the job of the routes.rb file. Exam- 
ple 7-3 is a routes.rb that sets up URIs for the six controllers I'll implement later in the 
chapter. 

Example 7-3. The routes.rb file 
# service/config/routes.rb 

ActionController : : Routing : : Routes . draw do | map | 
base = '/mi' 

## The first controller I define is the UsersController. The call to 
## map. resources sets it up so that all HTTP requests to /vl/users 
## or /vl/users/{username} are routed to the UsersController class. 

# /vl/users => UsersController 

map. resources :users, :path_prefix => base 

## Now I'm going to define a number of controllers beneath the 
## UsersController. They will respond to requests for URIs that start out 
## with /vl/users/{username}, and then have some extra stuff. 
user_base = base + '/users/:username' 

# /vl/users/{username}/bookmarks => BookmarksController 
map. resources :bookmarks, :path_prefix => user_base 

# /vl/users/{username}/tags => TagsController 
map. resources :tags, :path_prefix => user_base 

# /vl/users/{username}/calendar => CalendarController 
map. resources : calendar, :path_prefix => user_base 

## Finally, two more controllers that are rooted beneath /vl. 

# /vl/recent => RecentController 

map. resources :recent, :path_prefix => base 

# /vl/uris => UrisController 

map. resources :uris, :path_prefix => base 
end 

Now I'm committed to defining six controller classes. The code in Example 7-3 deter- 
mines the class names by tying into Rails' naming conventions. My six classes are called 
UsersController, BookmarksController, TagsController, CalendarController, 
RecentController, and UrisController. Each class controls one or two kinds of resour- 
ces. Each controller implements a specially-named Ruby method for each HTTP meth- 
od the resources expose. 



182 I Chapter 7: A Service Implementation 



Design the Representation(s) Accepted from the Client 

When a client wants to modify a user account or post a bookmark, how should it convey 
the resource state to the server? Rails transparently supports two incoming represen- 
tation formats: form-encoded key-value pairs and the ActiveRecord XML serialization 
format. 

Form-encoding should be familiar to you. I mentioned it back in Chapter 6, and it's 
everywhere in web applications. It's the q=jellyfish and colorl=blue&color2=green 
you see in query strings on the human web. When a client makes a request that includes 
the query string colorl=blue&color2=green, Rails gives the controller a hash that looks 
like this: 

{"colon" => "blue", "color2" => "green"} 

The service author doesn't have to parse the representation: they can work directly with 
the key-value pairs. 

ActiveRecord is Rails's object-relational library. It gives a native Ruby interface to the 
tables and rows in a relational database. In a Rails application, most exposed resources 
correspond to these ActiveRecord tables and rows. That's the case for my service: all 
my users and bookmarks are database rows managed through ActiveRecord. 

Any ActiveRecord object, and the database row that underlies it, can be represented as 
a set of key -value pairs. These key- value pairs can be form-encoded, but ActiveRecord 
also knows how to encode them into XML documents. Example 7-4 gives an XML 
depiction of an ActiveRecord object from this chapter: a user account. This is the string 
you'd get by calling to_xml on a (yet-to-be-defined) User object. Example 7-5 gives an 
equivalent form-encoded representation. Example 7-6 gives the hash that's left when 
Rails parses the XML document or the form-encoded string as an incoming 
representation. 

Example 7-4. An XML representation of a user account 
<user> 

<name>leonardr</body> 
<full-name> Leonard Richardson</body> 
<email>leonardr@example . com</body> 
<password>mypassword</body> 
</user> 

Example 7-5. A form-encoded representation of a user account 

user [name] =leonardr&user [full-name] =Leonard%20Richardson 
Suser [email] =leonardr%40example.com&user[ password] =mypassword 

Example 7-6. A set of key-value pairs derived from XML or the form-encoded representation 

{ "user[name]" => "leonardr", 

"user[full_name] " => "Leonard Richardson", 
"user[email] " => "leonardr@example.com", 
"user[password]" => "mypassword" } 



Design the Representation(s) Accepted from the Client | 183 



I'm going to support both representation formats. I can do this by defining my keys for 
the form-encoded representation as user[name] instead of just name. This looks a little 
funny to the client, but it means that Rails will parse a form-encoded representation 
and an ActiveRecord XML representation into the same data structure: one that looks 
like the one in Example 7-6. 

The keys for the key-value pairs of a user account representation are user [name], 
user[password], user[full_name], and user[email]. Not coincidentally, these are the 
names of the corresponding fields in my database table users. 

The keys for a representation of a bookmark are bookmark[short_description], 
bookmark[long_description], bookmark[timestamp], bookmark[public], and book 
mark[tag][]. These are all the names of database fields, except for bookmark[tag] [], 
which corresponds to a bookmark's tags. I'll be handling tags specially, and you might 
recall they're kept in separate database tables. For now, just note that the extra "[]" in 
the variable name tells Rails to expect multiple tags in a single request. 

<r * 

*• - I There are other ways of allowing the client to specify multiple tags. The 
*,f, del.icio.us service itself represents a list of tags as a single tags variable 
TLv containing a space-separated string. This is good for a simple case, but 
in general I don't like that because it reimplements something you can 
already do with the form-encoded format. 

A JSON data structure is another possible way of representing a book- 
mark. This would be a hash in which most keys correspond to strings, 
but where one key (tags) corresponds to a list. 

The incoming representation of a tag contains only one key-value pair: the key is 
tag[name]. 

The incoming representation of a bundle contains two key-value pairs: bundle[name] 
and bundle[tag] []. The second one can show up multiple times in a single represen- 
tation, since the point is to group multiple tags together. I'm approaching the imple- 
mentation stage, so this is the last time I'll mention bundles. 

Design the Representation(s) Served to the Client 

I've got a huge number of options for outgoing representation formats: think back to 
the discussion in "Representing the List of Planets" in Chapter 5. Rails makes it easy 
to serve any number of representation formats, but the simplest to use is the XML 
representation you get when you call to_xml on an ActiveRecord object. 

This is a very convenient format to serve from Rails, but it's got a big problem: it's not 
a hypermedia format. A client that gets the user representation in Example 7-4 knows 
enough to reconstruct the underlying row in the users table (minus the password) . But 
that document says nothing about the relationship between that resource and other 



184 | Chapter 7: A Service Implementation 



resources: the user's bookmarks, tag vocabulary, or calendar. It doesn't connect the 
"user" resource to any other resources. A service that serves only ActiveRecord XML 
documents isn't well-connected. 

I'm going to serve to_xml representations in a couple places, just to keep the size of this 
chapter down. I'll represent a user account and a user's tag vocabulary with to_xml. I'll 
generate my own, custom to_xml-like document when representing a user's posting 
history. 

When I think about the problem domain, another representation format leaps out at 
me: the Atom syndication format. Many of the resources I'm exposing are lists of 
bookmarks: recent bookmarks, bookmarks for a user, bookmarks for a tag, and so on. 
Syndication formats were designed to display lists of links. What's more, there are 
already lots of software packages that understand URIs and syndication formats. If I 
expose bookmark lists through a standard syndication format, I'll immediately gain a 
huge new audience for my service. Any program that manipulates syndication feeds 
can take my resources as input. What's more, syndication feeds can contain links. If a 
resource can be represented as a syndication feed, I can link it to other resources. My 
resources will form a web, not just an unrelated set. 

My default representation will always be the to_xml one, but a client will be able to get 
an Atom representation of any list of bookmarks by tacking ".atom" onto the end of 
the appropriate URI. If a client GETs /users/leonardr/bookmarks/ruby, it'll see a link- 
less to_xml representation of the bookmarks belonging to the user "leonardr" and tag- 
ged with "ruby." The URI /users/leonardr/bookmarks/ruby. atom will give an Atom 
representation of the same resource, complete with links to related resources. 

Connect Resources to Each Other 

There are many, many relationships between my resources. Think about the relation- 
ship between a user and her bookmarks, between a bookmark and the tags it was posted 
under, or between a URI and the users who've bookmarked it. But a to_xml represen- 
tation of a resource never links to the URI of another resource, so I can't show those 
relationships in my representations. On the other hand, an Atom feed can contain links, 
and can capture relationships between resources. 

Figure 7-1 shows my problem. When I think about the bookmarking service, I envision 
lots of conceptual links between the resources. But links only exist in the actual service 
when they're embodied in representations. Atom representations contain lots of links, 
but to_xml documents don't. To give one example, the conceptual link between a user 
and the user's bookmarks doesn't actually exist in my service. A client is just supposed 
to "know" how to get a user's bookmarks. 

Also note that while the "user" resource is clearly the focal point of the service, neither 
diagram gives any clue as to how a client can get to that resource in the first place. I've 



Connect Resources to Each Other | 185 




A user's 
calendar 



A particular 
bookmark 



/I 



A user 



A user's 
bookmark 




A user's tag 
vocabulary 



The 
Web 




Figure 7-1. The bookmarking service in my head versus the actual service 

described that in English prose. That means that my real audience is the people writing 
the web service clients, not the clients themselves. 

This is a failure of connectivity, and it's the same failure you can see in Amazon S3 and 
some other RESTful services. As REST becomes more popular, this kind of failure will 
probably be the last remaining vestige of the RPC style. I dealt with this problem in 
Chapter 5 by defining a service home page that linked to a few top-level resources. 
These resources linked to more resources, and so on. My fantasy map application was 
completely connected. 



What's Supposed to Happen? 

Rails exposes every database-backed application using only two resource patterns: lists 
(the database tables) and list items (the rows in a table). All list resources work pretty 
much the same way, as do all list item resources. Every "creation" operation follows 
the same rules and has similar failure conditions, whether the database row being cre- 
ated is a user, a bookmark, or something else. I can consider these rules as a sort of 
generic control flow, a set of general guidelines for implementing the HTTP interface 
for list and list item resources. I'll start defining that control flow here, and pick it up 
again in Chapter 9. 

When a resource is created, the response code should be 201 ("Created") and the 
Location header should point the way to the resource's location. 

When a resource is modified, the response code should be 200 ("OK"). If the resource 
state changes in a way that changes the URI to the resource (for instance, a user account 
is renamed), the response code is 301 ("Moved Permanently") and the Location header 
should provide the new URI. 

When an object is deleted, the response code should be 200 ("OK"). 



186 | Chapter 7: A Service Implementation 



As far as possible, all resources that support GET should also support conditional GET. 
This means setting appropriate values for ETag and Last-Modified. 

One final rule, a rule about data security. Unlike the del.icio.us API, I don't require 
authentication just to get information from the service. However, I do have a rule that 
no one should see a user's private bookmarks unless they're authenticated as that user. 
If you look at someone else's bookmarks, you'll get a representation that has her private 
bookmarks filtered out. You won't see the full resource state: just the part you're au- 
thorized to see. This principle extends past the bookmark lists themselves, and into 
things like the calendar and tag vocabulary. You should not see mysterious tags showing 
up in the representation of my tag vocabulary, tags that don't correspond to any of the 
tags I used in my visible bookmarks. This last rule is specific to my social bookmarking 
application, but its lessons can be applied more generally. 

What Might Go Wrong? 

The main problem is unauthorized access. I can use the 401 response code ("Unau- 
thorized") any time the client tries to do something (edit a user's account, rename a tag 
for a user) without providing the proper Authorization header. 

A client might try to create a user account that already exists. From the point of view 
of the service, this looks like an attempt to modify the existing account without pro- 
viding any authorization. The response code of 401 ("Unauthorized") is appropriate, 
but it might be confusing to the client. My service will send a 401 response code when 
the authorization is provided but incorrect, and a 409 ("Conflict") when no authori- 
zation at all is provided. This way, a client who thought she was creating a new account 
is less likely to be confused. 

Similarly, a client might try to rename a user account to a name that already exists. The 
409 response code is appropriate here as well. 

Any resource that's a list of bookmarks will support query variables limit and date. 
These variables place restrictions on which bookmarks should show up in the repre- 
sentation: the client can set a maximum number of bookmarks to retrieve, or restrict 
the operation to bookmarks posted on a certain date. If the client sends a nonsensical 
limit or date, the appropriate response code is 400 ("Bad Request"). I'll also use 400 
when a user tries to create or modify a resource, but doesn't provide a valid 
representation. 

If the client tries to retrieve information about a nonexistent user, this service will do 
what del.icio.us does and send a response code of 404 ("Not Found") . This is the client's 
cue to create that user account if they wish. I'll do the same if the client tries to get 
information about a URI that no one has bookmarked. 

A user can modify the URI listed in one of her bookmarks, but she can only have one 
bookmark for a given URI. If a user tries to change a bookmark's URI to one she's 



What Might Go Wrong? | 187 



already bookmarked, a response code of 409 ("Conflict") is appropriate. 409 is also the 
correct response if the user tries to POST a URI she's already bookmarked. The uniform 
way to modify an existing bookmark is with PUT on the bookmark resource. 

If the client tries to create a user account or bookmark, but provides an incomplete or 
nonsensical representation, the response is 400 ("Bad Request"). For instance, the cli- 
ent might try to POST a new bookmark, but forget to send the URI of the bookmark. 
Or it might try to bookmark a "URI" that's not a URI at all. 

When creating a user, the client might send a JSON representation of a new user, 
instead of an ActiveRecord XML or form-encoded representation of the same data. In 
other words, it might send the totally wrong media type. The proper response code 
here is 415 ("Unsupported Media Type"). Rails handles this failure condition 
automatically. 

Controller Code 

Now we come to the heart of the application: the code that converts incoming HTTP 
requests into specific actions on the database. I'm going to define a base class called 
ApplicationController, which contains common code, including almost all of the 
tricky code. Then I'll define the six controller classes I promised earlier. 

Each controller class will implement some actions: methods that are called to handle 
a HTTP request. Rails defines a list of standard actions that correspond to methods 
from HTTP's uniform interface. I mentioned these earlier: the index action is invoked 
in response to GET for a "list" type resource, and so on. Those are the actions I'll be 
defining, though many of them will delegate to other actions with nonstandard names. 

There's a lot of code in this application, but relatively little of it needs to be published 
in this book. Most of the low-level details are in Rails, the plugins, and the atom- 
tools gem. I can express my high-level ideas almost directly in code. Of course, my 
reliance on external code occasionally has downsides, like the fact that some of my 
representations don't contain links. 

What Rails Doesn't Do 

There's one feature I want for my service that isn't built into Rails or plugins, and there's 
another that goes against Rails's path of least resistance. I'm going to be implementing 
these features myself. These two items account for much of the tricky code in the 
service. 

Conditional GET 

Wherever possible, a web service should send the response headers Last-Modified and 
ETag along with a representation. If the client makes future requests for the same re- 
source, it can make its requests conditional on the representation having changed since 



188 | Chapter 7: A Service Implementation 



the last GET. This can save time and bandwidth; see "Conditional GET" in Chap- 
ter 8 for more on this topic. 

There are third-party Rails controllers that let the programmer provide values for Last- 
Modif ied and ETag. Core Rails doesn't do this, and I don't want to bring in the additional 
complexity of a third-party controller. I implement a fairly reusable solution for Last- 
Modified in Example 7-9. 

param[:id] for things that aren't IDs 

Rails assumes that resources map to ActiveRecord objects. Specifically, it assumes that 
the URI to a "list item" resource identifies a row in a database table by ID. For instance, 
it assumes the client will request the URI /vl/users/4 instead of the more readable 
URI /vl/users/leonardr. 

The client can still request /users/leonardr, and the controller can still handle it. This 
just means that the username will be available as params[:id] instead of something 
more descriptive, like params[ : username] . 

If a URI contains more than one path variable, then when I define that URI in 
routes.rb I get to choose the params name for all but the last one. The last variable always 
gets put into params [ :id], even if it's not an ID. The URI /vl/users/leonardr/tags/ 
food has two path variables, for example. params[ :username], named back in Exam- 
ple 7-3, has a value of "leonardr". The tag name is the one that gets put into par 
ams[:id]. I'd rather call it params[ :tag], but there's no good way to do that in Rails. 
When you see params [ : id] in the code below, keep in mind that it's never a database ID. 

The ApplicationController 

This class is the abstract superclass of my six controllers, and it contains most of the 
common functionality (the rest will go into the ActiveRecord model classes). Exam- 
ple 7-7 starts by defining an action for the single most common operation in this service: 
fetching a list of bookmarks that meet some criteria. 

Example 7-7. app/controllers/application.rb 

# app/controllers/application.rb 
require 'digest/shal' 
require 'digest/mds' 
require 'rubygems' 
require 'atom/feed' 

class ApplicationController < ActionController: :Base 

# By default, show 50 bookmarks at a time. 
(5)@default_limit = 50 

## Common actions 

# This action takes a list of SQL conditions, adds some additional 



Controller Code | 189 



# conditions like a date filter, and renders an appropriate list of 

# bookmarks. It's used by BookmarksController, RecentController, 

# and TagsController. 

def show_bookmarks(conditions, title, feed_uri, user=nil, tag=nil) 
errors = [] 

# Make sure the specified limit is valid. If no limit is specified, 

# use the default. 

if params[ :limit] && params[ : limit ] .to_i < 

errors << "limit must be >=0" 
end 

params[ :limit] ||= @@default_limit 

params.delete( :limit) if params[ :limit] ==0 #0 means "no limit" 

# If a date filter was specified, make sure it's a valid date, 
if params[ :date] 

begin 

params[ :date] = Date.parse(params[ :date]) 
rescue ArgumentError 

errors << "incorrect date format" 
end 
end 

if errors. empty? 
conditions | | = [""] 

# Add a restriction by date if necessary, 
if params[ :date] 

conditions[o] << " AND " unless conditions[o] .empty? 
conditions[o] << "timestamp >= ? AND timestamp < ?" 
conditions << params[ :date] 
conditions << params[ :date] + 1 
end 

# Restrict the list to bookmarks visible to the authenticated user. 
Bookmark. only_visible_to! (conditions, (5>authenticated_user) 

# Find a set of bookmarks that matches the given conditions, 
bookmarks = Bookmark. custom_find(conditions, tag, params[ : limit]) 

# Render the bookmarks however the client requested. 
render_bookmarks(bookmarks, title, feed_uri, user) 

else 

render :text => errors. join("\n"), :status => "400 Bad Request" 
end 
end 

The show_bookmarks method works like any Rails action: it gets query parameters like 
limit from params, and verifies them. Then it fetches some data from the database and 
renders it with a view. A lot of my RESTful action methods will delegate to this method. 
If the RESTful action specifies no conditions, show_bookmarks will fetch all the book- 
marks that match the date and tag filters, up to the limit. Most of my actions will impose 
additional conditions, like only fetching bookmarks posted by a certain user. 



190 | Chapter 7: A Service Implementation 



The main difference between show_bookmarks and a traditional Rails action is in the 
view. Most Rails actions define the view with an ERb template like show.rhtml: a com- 
bination of HTML and Ruby code that works like JSP templates or PHP code. Instead, 
I'm passing the job off to the render_bookmarks function (see Example 7-8). This func- 
tion uses code-based generators to build the XML and Atom documents that serve as 
representations for most of my application's resources. 

Example 7-8. application.rb continued: render _bookmarks 

# This method renders a list of bookmarks as a view in RSS, Atom, or 

# ActiveRecord XML format. It's called by show_bookmarks 

# above, which is used by three controllers. It's also used 

# separately by UriController and BookmarksController. 
# 

# This view method supports conditional HTTP GET. 

def render_bookmarks(bookmarks, title, feed_uri, user, except=[]) 

# Figure out a current value for the Last-Modified header, 
if bookmarks. empty? 

last_modified = nil 
else 

# Last-Modified is the most recent timestamp in the bookmark list. 
most_recent_bookmark = bookmarks. max do |bl,b2| 

bl. timestamp <=> b2. timestamp 
end 

lastjnodified = most_recent_bookmark. timestamp 
end 

# If the bookmark list has been modified since it was last requested... 
render_not_modified_or( lastjnodified) do 

respond_to do | format | 

# If the client requested XML, serialize the ActiveRecord 

# objects to XML. Include references to the tags in the 

# serialization. 

format. xml { render :xml => 
bookmarks. to_xml (: except => except + [:id, :user_id], 
:include => [:tags]) } 

# If the client requested Atom, turn the ActiveRecord objects 

# into an Atom feed. 

format. atom { render :xml => atom_feed_for( bookmarks, title, 

feed_uri, user) } 

end 
end 
end 

That method is also where I start handling conditional HTTP requests. I've chosen to 
use the timestamp of the most recent bookmark as the value of the HTTP header Last - 
Modified. 

The rest of the conditional request handling is in the render_not_modified_or function 
(see Example 7-9). It's called just before render_bookmarks is about to write the list of 
bookmarks, and it applies the rules of conditional HTTP GET. If the list of bookmarks 
has changed since this client last requested it, this function calls the Ruby keyword 
yield and the rest of the code in render_bookmarks runs normally. If the list of book- 



Controller Code | 191 



marks hasn't changed, this function short-circuits the Rails action, sending a response 
code of 304 ("Not Modified") instead of serving the representation. 

Example 7-9. application.rb continued: render _not_modified_or 
## Helper methods 

# A wrapper for actions whose views support conditional HTTP GET. 

# If the given value for Last-Modified is after the incoming value 

# of If-Modified-Since, does nothing. If Last-Modified is before 

# If-Modified-Since, this method takes over the request and renders 

# a response code of 304 ("Not Modified"), 
def render_not_modified_or(last_modified) 

response. headers [' Last-Modified' ] = lastjnodified. httpdate if lastjnodified 

if_modified_since = request. env[ 'HTTP_IF_MODIFIED_SINCE' ] 
if if_modified_since && lastjnodified && 

lastjnodified <= Time.httpdate(if_modified_since) 

# The representation has not changed since it was last requested. 

# Instead of processing the request normally, send a response 

# code of 304 ("Not Modified"). 

render : nothing => true, : status => "304 Not Modified" 
else 

# The representation has changed since it was last requested. 

# Proceed with normal request processing, 
yield 

end 
end 

Example 7-10 shows one more helper function used in multiple actions. The 
ifjfound method makes sure the client specified a URI that corresponds to an object 
in the database. If given a non-null object, nothing happens: if jf ound uses yield to 
return control to the action that called it. If given a null object, the function short- 
circuits the request with a response code of 404 ("Not Found"), and the action never 
gets a chance to run. 

Example 7-10. application.rb continued: ijjound. 

# A wrapper for actions which require the client to have named a 

# valid object. Sends a 404 response code if the client named a 

# nonexistent object. See the userjid_from_username filter for an 

# example. 

def ifjfound(obj) 
if obj 

yield 
else 

render :text => "Not found.", :status => "404 Not Found" 
false 
end 
end 

I've also implemented a number ol filters: pieces of code that run before the Rails actions 
do. Some Rails filters perform common setup tasks (see Example 7-11). This is the job 
of authenticate, which checks the client's credentials. Filters may also check for a 



192 | Chapter 7: A Service Implementation 



problem and short-circuit the request if they find one. This is the job of 
must_authenticate, and also must_specify_user, which depends on the if_found meth- 
od defined above. Filters let me keep common code out of the individual actions. 

Example 7-11. application. rb continued: filters 
## Filters 

# All actions should try to authenticate a user, even those actions 

# that don't require authorization. This is so we can show an 

# authenticated user their own private bookmarks. 
before_filter authenticate 

# Sets @authenticated_user if the user provides valid 

# credentials. This may be used to deny access or to customize the 

# view. 

def authenticate 
@authenticated_user = nil 
authenticate_with_http_basic do |user, pass| 

@authenticated_user = User.authenticated_user(user, pass) 
end 

return true 
end 

# A filter for actions that _require_ authentication. Unless the 

# client has authenticated as some user, takes over the request and 

# sends a response code of 401 ("Unauthorized"). Also responds with 

# a 401 if the user is trying to operate on some user other than 

# themselves. This prevents users from doing things like deleting 

# each others' accounts, 
def must_authenticate 

if @authenticated_user && (lg)user_is_viewing_themselves != false) 

return true 
else 

request_http_basic_authentication ( "Social bookmarking service" ) 
return false 
end 
end 

# A filter for controllers beneath /users/{username}. Transforms 

# {username} into a user ID. Sends a 404 response code if the user 

# doesn't exist. 

def must_specify_user 
if paramsf :username] 

@user = User.find_by_name(params[ :username] ) 

if_found(@user) { params[ :user_id] = (Suser.id } 

return false unless (Buser 
end 

@user_is_viewing_themselves = ((5>authenticated_user == (Buser) 
return true 
end 

Finally, the application controller is where I'll implement my primary view method: 
atom_feed_for (see Example 7-12). This method turns a list of ActiveRecord Bookmark 



Controller Code | 193 



objects into an Atom document. The controller that wants to serve a list of bookmarks 
needs to provide a title for the feed (such as "Bookmarks for leonardr") and a URI to 
the resource being represented. The resulting document is rich in links. Every book- 
mark links to the external URI, to other people who bookmarked that URI, and to 
bookmarks that share tags with this one. 

Example 7-12. application. rb concluded: atomjeedjor 
## Methods for generating a representation 

# This method converts an array of ActiveRecord ' s Bookmark objects 

# into an Atom feed. 

def atom_feed_for(bookmarks, title, feed_uri, user=nil) 
feed = Atom: : Feed. new 
feed. title = title 

most_recent_bookmark = bookmarks. max do |bl,b2| 

bl.timestamp <=> b2.timestamp 
end 

feed. updated = most_recent_bookmark.timestamp 

# Link this feed to itself 
self_link = feed. links. new 
self_link['rel'] = 'self 
self_link[ ' href ' ] = feed_uri + ".atom" 

# If this list is a list of bookmarks from a single user, that user is 

# the author of the feed, 
if user 

user_to_atom_author(user, feed) 
end 

# Turn each bookmark in the list into an entry in the feed, 
bookmarks. each do | bookmark | 

entry = feed. entries. new 

entry. title = bookmark. short_description 

entry. content = bookmark. long_description 

# In a real application, a bookmark would have a separate 

# "modification date" field which was not under the control of 

# the user. This would also make the Last-Modified calculations 

# more accurate. 

entry. updated = bookmark. timestamp 

# First, link this Atom entry to the external URI that the 

# bookmark tracks. 
external_uri = entry. links. new 
external_uri[ ' href ' ] = bookmark. uri 

# Now we give some connectedness to this service. Link this Atom 

# entry to this service's resource for this bookmark. 
bookmark_resource = entry. links. new 
bookmark_resource[ 'rel' ] = "self" 

bookmark_resource[ ' href ' ] = bookmark_url (bookmark. user. name, 

bookmark. uri_hash) + ".atom" 
bookmark_resource[ 'type' ] = "application/xml+atom" 



194 | Chapter 7: A Service Implementation 



# Then link this entry to the list of users who've bookmarked 

# this URI. 

other_users = entry. links. new 
other_users[ 'rel' ] = "related" 

other_users[ ' href ' ] = uri_url(bookmark.uri_hash) + ".atom" 
other_users[ 'type' ] = "application/xml+atom" 

# Turn this entry's user into the "author" of this entry, unless 

# we already specified a user as the "author" of the entire 

# feed, 
unless user 

user_to_atom_author (bookmark. user, entry) 
end 

# For each of this bookmark's tags... 
bookmark. tags. each do |tag| 

# ...represent the tag as an Atom category, 
category = entry. categories. new 
category[ 'term' ] = tag 

category[ ' scheme' ] = user_url(bookmark. user. name) + "/tags" 

# Link to this user's other bookmarks tagged using this tag. 
tag_uri = entry. links. new 

tag_uri[ ' href ' ] = tag_url(bookmark. user. name, tag. name) + ".atom" 

tag_uri[ 'rel' ] = 'related' 

tag_uri[ 'type' ] = "application/xml+atom" 

# Also link to all bookmarks tagged with this tag. 
recent_tag_uri = entry. links. new 

recent_tag_uri['href] = recent_url(tag.name) + ".atom" 
recent_tag_uri[ 'rel' ] = 'related' 
recent_tag_uri[ 'type' ] = "application/xml+atom" 
end 
end 

return feed. to xml 



# Appends a representation of the given user to an Atom feed or element 
def user_to_atom_author(user, atom) 

author = atom. authors. new 

author. name = user.full_name 

author. email = user. email 

author. uri = user_url(user.name) 
end 
end 

Example 7-13 shows what kind of Atom representation this method might serve. 

Example 7-13. An Atom representation of a list of bookmarks 

<feed xmlns=' http://www.w3.org/2005/Atom' > 
<title>Bookmarks for leonardr</title> 
<screen> 

<updated>2007-02-14T02 : 26 : 58-05 : 00</updated> 

<link href ="http://localhost:3000/vl/users/leonardr/bookmarks. atom" rel="self"/> 



Controller Code | 195 



<author> 

<name>leonardr</name> 

<uri>http : //localhost : 3000/vl/users/leonardr</uri> 
<email>leonardr(5>example . com</email> 
</author> 

<entry> 

<title>REST and WS-*/title> 

<content>]oe Gregorio's lucid explanation of RESTful principles</content> 
<category term="rest" scheme="http: //localhost :3000/vl/users/leonardr/rest"/> 
<link href =" http://bitworking.org/news/i25/REST-and-WS" rel= "alternate" /> 

<link href ="http://localhost:3000/vl/users/leonardr/bookmarks/68044f26e373de4a08ff343a7fa5f675. atom" 
rel="self" type="application/xml+atom"/> 

<link href ="http : / /localhost : 3000/vl/recent/ rest . atom" 
rel="related" type="application/xml+atom"/> 
<updated>2007-02-14T02 :26: 58-05 :00</updated> 
</entry> 
</feed> 

The UsersController 

Now I'm ready to show you some specific actions. I'll start with the controller that 
makes user accounts possible. In the code in Example 7-14, note the call to 
bef ore_f ilter that sets up the must_authenticate filter. You don't need to authenticate 
to create (POST) a user account (as whom would you authenticate?), but you must 
authenticate to modify (PUT) or destroy (DELETE) an account. 

Example 7-14. app/controllers/users_controller.rb 

class UsersController < ApplicationController 

# A client must authenticate to modify or delete a user account. 
before_filter :must_authenticate, :only => [:modify, :destroy] 

# POST /users 
def create 

user = User.find_by_name(params[ :user] [ :name] ) 
if user 

# The client tried to create a user that already exists, 
headersf ' Location' ] = user_url(user.name) 
render : nothing => true, : status => "409 Conflict" 
else 

user = User.new(params[ :user]) 
if user. save 

headers [' Location' ] = user_path(user.name) 

render : nothing => true, : status => "201 Created" 
else 

# There was a problem saving the user to the database. 

# Send the validation error messages along with a response 

# code of 400. 

render :xml => user. errors. to_xml, :status => "400 Bad Request" 
end 



196 I Chapter 7: A Service Implementation 



end 
end 



The conventions of RESTful Rails impose a certain structure on UsersController (and, 
indeed, on the name of the class itself). This controller exposes a resource for the list 
of users, and one resource for each particular user. The create method corresponds to 
a POST to the user list. The show, update, and delete methods correspond to a GET, 
PUT, or DELETE request on a particular user. 

The create method follows a pattern I'll use for POST requests throughout this service. 
If the client tries to create a user that already exists, the response code is 409 ("Con- 
flict"). If the client sends bad or incomplete data, the ActiveRecord validation rules 
(defined in the User) model) fail, and the call to User#save returns false. The response 
code then is 400 ("Bad Request"). If all goes well, the response code is 201 ("Created") 
and the Location header contains the URI of the newly created user. All I've done in 
Example 7-15 is put into code the things I said in "What's Supposed to Happen?" and 
"What Might Go Wrong?" earlier in this chapter. I'll mention this generic control flow 
again in Chapter 8. 

Example 7-15. app/controllers/users_controller.rb continued 

# PUT /users/{username} 
def update 

old_name = params[ :id] 

new_name = params[ : user] [: name] 

user = User.find_by_name(old_name) 

if_found user do 
if old_name != new_name && User.find_by_name(new_name) 

# The client tried to change this user's name to a name 

# that's already taken. Conflict! 

render : nothing => true, : status => "409 Conflict" 
else 

# Save the user to the database, 
user . update_attributes (params [ : user] ) 
if user. save 

# The user's name changed, which changed its URI. 

# Send the new URI. 

if user. name != old_name 

headersf ' Location' ] = user_path (user. name) 

status = "301 Moved Permanently" 
else 

# The user resource stayed where it was. 
status = "200 OK" 
end 

render : nothing => true, : status => status 
else 

# There was a problem saving the bookmark to the database. 

# Send the validation error messages along with a response 

# code of 400. 

render :xml => user. errors. to_xml, :status => "400 Bad Request" 
end 
end 



Controller Code | 197 



end 
end 

The update method has a slightly different flow, and it's a flow I'll use for PUT requests 
throughout the service. The general outline is the same as for POST. The twist is that 
instead of trying to create a user (whose name might already be in use), the client can 
rename an existing user (and their new name might already be in use). 

I send a 409 response code ("Conflict") if the client proposes a new username that 
already exists, and a 400 response code ("Bad Request") if the data validation errors 
fail. If the client successfully edits a user, I send not a 201 response code ("Created") 
but a simple 200 ("OK"). 

The exception is if the client successfully changes a user's name. Now that resource is 
available under a different URI: say, /users/leonard instead of /users/leonardr. That 
means I need to send a response code of 301 ("Moved Permanently") and put the user's 
new URI in the Location header. 

The GET and DELETE implementations are more straightforward, as shown in Ex- 
ample 7-16. 

Example 7-16. app/controllers/users_controller.rb continued 

# GET /users/{username} 
def show 

# Find the user in the database, 
user = User.find_by_name(params[ :id] ) 
if_found(user) do 

# Serialize the User object to XML with ActiveRecord ' s to_xml. 

# Don't include the user's ID or password when building the XML 

# document. 

render :xml => user.to_xml( :except => [:id, :password]) 
end 
end 

# DELETE /users/{username} 
def destroy 

user = User.find_by_name(params[ :id] ) 
if_found user do 

# Remove the user from the database, 
user. destroy 

render : nothing => true, : status => "200 OK" 
end 
end 
end 

There is one hidden detail: the if_found method sends a response code of 404 ("Not 
Found") if the user tries to GET or DELETE a nonexistent user. Otherwise, the response 
code is 200 ("OK"). I have not implemented conditional HTTP GET for user resources: 
I figured the possible bandwidth savings wasn't big enough to justify the added 
complexity. 



198 | Chapter 7: A Service Implementation 



The BookmarksController 

This is the other main controller in this application (see Example 7-17). It exposes a 
user's list of bookmarks and each individual bookmark. The filters are interesting here. 
This BookmarksController is for displaying a particular user's bookmarks, and any at- 
tempt to see a nonexistent user's bookmarks should be rebuffed with a stern 404 ("Not 
Found"). That's the job of the must_specify_user filter I defined earlier. The 
must_authenticate filter works like it did in UsersController: it prevents unauthenti- 
cated requests from getting through to Rails actions that require authentication. I've 
also got a one-off filter, f ix_params, that enforces consistency in incoming representa- 
tions of bookmarks. 

Example 7-17. app/controllers/bookmarks_controller.rb 

class BookmarksController < ApplicationController 
before_f liter :must_specify_user 
before_filter :fix_params 

before_filter :must_authenticate, :only => [:create, :update, :destroy] 

# This filter cleans up incoming representations, 
def fix_params 

if params[ : bookmark] 

params[ :bookmark] [ :user_id] = @user.id if @user 
end 
end 

The rest of BookmarksController is just like UsersController: fairly involved create 
(POST) and update (PUT) methods, simple show (GET) and delete (DELETE) methods 
(see Example 7-18). The only difference is that this controller's list resource responds 
to GET, so I start with a simple implementation of index. Like many of the Rails actions 
I'll define, index and show simply delegate to the show_bookmarks action. 

Example 7-18. app/controllers/bookmarks_controller.rb continued 

# GET /users/{username}/bookmarks 
def index 

# Show this user's bookmarks by passing in an appropriate SQL 

# restriction to show_bookmarks. 
show_bookmarks(["user_id = ?", (ffluser.id], 

"Bookmarks for #{@user.name}", 
bookmark_url((5>user.name), @user) 

end 

# POST /users/{username}/bookmarks 
def create 

bookmark = Bookmark. f ind_by_user_id_and_uri ( params [: bookmark] [ :user_id] , 

params [ : bookmark] [ : uri] ) 

if bookmark 

# This user has already bookmarked this URI. They should be 

# using PUT instead. 

headers [' Location' ] = bookmark_url((5>user.name, bookmark. uri) 
render : nothing => true, : status => "409 Conflict" 
else 



Controller Code | 199 



# Enforce default values for 'timestamp' and 'public' 
params[ :bookmark] [ :timestamp] ||= Time. now 

params[ :bookmark] [ :public] ||= "1" 

# Create the bookmark in the database, 
bookmark = Bookmark. new(params[ :bookmark]) 
if bookmark. save 

# Add tags. 

bookmark. tag_with(params[ :taglist]) if params[ :taglist] 

# Send a 201 response code that points to the location of the 

# new bookmark. 

headers [' Location' ] = bookmark_url((5)user.name, bookmark. uri) 
render : nothing => true, : status => "201 Created" 
else 

render :xml => bookmark. errors. to_xml, :status => "400 Bad Request" 
end 
end 
end 

# PUT /users/{username}/bookmarks/{URI-MD5} 
def update 

bookmark = Bookmark. find_by_user_id_and_uri_hash((5>user. id, params[:id]) 
if_found bookmark do 

old_uri = bookmark. uri 

if old_uri != params[ :bookmark] [ :uri] && 

Bookmark. find_by_user_id_and_uri((8user. id, params[ :bookmark] [ :uri]) 

# The user is trying to change the URI of this bookmark to a 

# URI that they've already bookmarked. Conflict! 
render : nothing => true, : status => "409 Conflict" 

else 

# Update the bookmark's row in the database. 

if bookmark . update_attributes (params [ : bookmark] ) 
# Change the bookmark's tags. 

bookmark. tag_with (params [ :taglist]) if params[ :taglist] 
if bookmark. uri != old_uri 

# The bookmark changed URIs. Send the new URI. 

headers [' Location' ] = bookmark_url((5>user.name, bookmark. uri) 
render : nothing => true, : status => "301 Moved Permanently" 
else 

# The bookmark stayed where it was. 

render : nothing => true, : status => "200 OK" 
end 
else 

render :xml => bookmark. errors. to_xml, :status => "400 Bad Request" 
end 
end 
end 
end 

# GET /users/{username}/bookmarks/{uri} 
def show 

# Look up the requested bookmark, and render it as a "list" 

# containing only one item. 

bookmark = Bookmark. find_by_user_id_and_uri_hash((5>user. id, params[:id]) 



200 | Chapter 7: A Service Implementation 



if_found(bookmark) do 
render_bookmarks ( [ bookmark] , 

"#{(5>user.name} bookmarked #{bookmark.uri}", 
bookmark_url(@user.name, bookmark. uri_hash), 
@user) 

end 
end 

# DELETE /users/{username}/bookmarks/{uri} 
def destroy 

bookmark = Bookmark. find_by_user_id_and_uri_hash((5>user. id, params[:id]) 
if_found bookmark do 
bookmark. destroy 

render : nothing => true, : status => "200 OK" 
end 
end 
end 

The TagsController 

This controller exposes a user's tag vocabulary, and the list of bookmarks she's filed 
under each tag (see Example 7-19). There are two twists here: the tag vocabulary and 
the "tag rename" operation. 

The tag vocabulary is simply a list of a user's tags, along with a count of how many 
times this user used the tag. I can get this data fairly easily with ActiveResource, and 
format it as a representation with to_xml, but what about security? If you tag two public 
and six private bookmarks with "ruby," when I look at your tag vocabulary, I should 
only see "ruby" used twice. If you tag a bunch of private bookmarks with "possible- 
acquisition," I shouldn't see "possible-acquisition" in your vocabulary at all. On the 
other hand, when you're viewing your own bookmarks, you should be able to see the 
complete totals. I use some custom SQL to count only the public tags when appropriate. 
Incidentally, this is another resource that doesn't support conditional GET. 

Example 7-19. app/controllers/tags_controller.rb 

class TagsController < ApplicationController 
before_filter :must_specify_user 
before_filter :must_authenticate, :only => [:update] 

# GET /users/{username}/tags 
def index 

# A user can see all of their own tags, but only tags used 

# in someone else's public bookmarks, 
if @user_is_viewing_themselves 

tag_restriction = ' ' 
else 

tag_restriction = " AND bookmarks. public='l"' 
end 

sql = ["SELECT tags.*, COUNT(bookmarks.id) as count" + 
" FROM tags, bookmarks, taggings" + 
" WHERE taggings. taggablejtype = 'Bookmark'" + 
" AND tags. id = taggings. tag_id" + 



Controller Code | 201 



" AND taggings.taggable_id = bookmarks. id" + 
" AND bookmarks. user_id = ?" + tag_restriction + 
" GROUP BY tags. name", (Buser.id] 

# Find a bunch of ActiveRecord Tag objects using custom SQL. 
tags = Tag.find_by_sql(sql) 

# Convert the Tag objects to an XML document, 
render :xml => tags. to_xml(: except => [:id]) 

end 

I said earlier I'd handle the "tag rename" operation with HTTP PUT. This makes sense 
since a rename is a change of state for an existing resource. The difference here is that 
this resource doesn't correspond to a specific ActiveRecord object. There's an Active- 
Record Tag object for every tag, but that object represents everyone's use of a tag. This 
controller doesn't expose tags, per se: it exposes a particular user's tag vocabulary. 

Renaming a Tag object would rename it for everybody on the site. But if you rename 
"good" to "bad," then that should only affect your bookmarks. Any bookmarks I've 
tagged as "good" should stay "good." The client is not changing the tag, just one user's 
use of the tag. 

From a RESTful perspective none of this matters. A resource's state is changed with 
PUT, and that's that. But the implementation is a bit tricky. What I need to do is find 
all the client's bookmarks tagged with the given tag, strip off the old tag, and stick the 
new tag on. Unlike with users or bookmarks, I won't be sending a 409 ("Conflict") 
response code if the user renames an old tag to a tag that already exists. I'll just merge 
the old tag into the new one (see Example 7-20) . 

Example 7-20. app/controllers/tags_controller.rb continued 

# PUT /users/{username}/tags/{tag} 

# This PUT handler is a little trickier than others, because we 

# can't just rename a tag site-wide. Other users might be using the 

# same tag. We need to find every bookmark where this user uses the 

# tag, strip the "old" name, and add the "new" name on. 
def update 

oldjiame = params[:id] 

new_name = params[ :tag] [ :name] if params[:tag] 
if new_name 

# Find all this user's bookmarks tagged with the old name 
to_change = Bookmark. find([ "bookmarks. user_id = ?", loiuser.id], 

oldjiame) 

# For each such bookmark... 
to_change.each do | bookmark | 

# Find its tags. 

tags = bookmark. tags. collect { |tag| tag. name } 

# Remove the old name. 
tags.delete(old_name) 

# Add the new name, 
tags << new_name 

# Assign the new set of tags to the bookmark, 
bookmark. tag_with tags.uniq 

end 



202 | Chapter 7: A Service Implementation 



headers [' Location' ] = tag_url(@user.name, newjiame) 
status = "301 Moved Permanently" 
end 

render : nothing => true, : status => status || "200 OK" 
end 

# GET /users/{username}/tags/{tag} 
def show 

# Show bookmarks that belong to this user and are tagged 

# with the given tag. 
tag = params[:id] 

show_bookmarks([ "bookmarks. user_id = ?", @user.id], 

"#{@user.name}'s bookmarks tagged with '#{tag}"', 
tag_url(@user.name, tag), @user, tag) 

end 
end 

The Lesser Controllers 

Every other controller in my application is read-only. This means it implements at most 
index and show. Hopefully by now you get the idea behind the controllers and their 
action methods, so I'll cover the rest of the controllers briefly. 

The CalendarController 

This resource, a user's posting history, is something like the one exposed by TagsCon 
troller#show. I'm getting some counts from the database and rendering them as XML. 
This document doesn't directly correspond to any ActiveRecord object, or list of such 
objects; it's just a summary. As before, I need to be sure not to include other peoples' 
private bookmarks in the count. 

The main body of code goes into the Bookmark. calendar method, defined in the Book 
mark model class (see "The Bookmark Model). The controller just renders the data. 

ActiveRecord's to_xml doesn't do a good job on this particular data structure, so I've 
implemented my own view function: calendar_to_xml (see Example 7-21). It uses 
Builder: :XmlHarkup (a Ruby utility that comes with Rails) to generate an XML docu- 
ment without writing much code. 

Example 7-21. app <i 'controllers/ 'calendar _contr oiler. rb 

class CalendarController < ApplicationController 
before_filter :must_specify_user 

# GET /users/{username}/calendar 
def index 

calendar = Bookmark. calendar(@user. id, (5)user_is_viewing_themselves) 
render :xml => calendar_to_xml(calendar) 
end 

# GET /users/{username}/calendar/{tag} 
def show 



Controller Code | 203 



tag = params[:id] 

calendar = Bookmark. calendar(@user. id, («)user_is_viewing_themselves, 

tag) 

render :xml => calendar_to_xml(calendar, tag) 
end 

private 

# Build an XML document out of the data structure returned by the 

# Bookmark. calendar method. 

def calendar_to_xml(days, tag=nil) 
xml = Builder: :XmlMarkup.new( :indent => 2) 
xml. instruct ! 

# Build a 'calendar' element. 
xml.calendar(:tag => tag) do 

# For every day in the data structure... 
days. each do | day | 
# ...add a "day" element to the document 
xml.day(:date => day. date, :count => day. count) 
end 
end 
end 
end 

The RecentController 

The controller in Example 7-22 shows recently posted bookmarks. Its actions are just 
thin wrappers around the show_bookmarks method defined in application. rb. 

Example 7-22. app/controllers/recent_controller.rb 
# recent_controller.rb 

class RecentController < ApplicationController 

# GET /recent 
def index 

# Take bookmarks from the database without any special conditions. 

# They'll be ordered with the most recently-posted first. 
show_bookmarks(nil, "Recent bookmarks", recent_url) 

end 

# GET /recent/{tag} 
def show 

# The same as above, but only fetch bookmarks tagged with a 

# certain tag. 
tag = params[:id] 

show_bookmarks(nil, "Recent bookmarks tagged with '#{tag}"', 
recent_url(tag), nil, tag) 

end 
end 



204 | Chapter 7: A Service Implementation 



The UrisController 

The controller in Example 7-23 shows what the site's users think of a particular URI. 
It shows a list of bookmarks, all for the same URI but from different people and with 
different tags and descriptions. 

Example 7-23. app/controllers/uris_controller.rb 
# uris_controller.rb 

class UrisController < ApplicationController 
# GET /uris/{URI-MD5} 
def show 

# Fetch all the visible Bookmark objects that correspond to 

# different people bookmarking this URI. 
uri_hash = params[ :id] 

sql = ["SELECT bookmarks.*, users. name as user from bookmarks, users" + 
" WHERE users. id = bookmarks. user_id AND bookmarks. uri_hash = ?", 
uri_hash] 

Bookmark. only_visible_to! (sql, (5>authenticated_user) 
bookmarks = Bookmark. find_by_sql( sql) 

if_found(bookmarks) do 

# Render the list of Bookmark objects as XML or a syndication feed, 

# depending on what the client requested, 
uri = bookmarks [o] .uri 

render_bookmarks(bookmarks, "Users who've bookmarked #{uri}", 
uri_url(uri_hash), nil) 

end 
end 
end 

Model Code 

Those are the controllers. I've also got three "model" classes, corresponding to my three 
main database tables: User, Bookmark, and Tag. The Tag class is defined entirely 
through the acts_as_taggable Rails plugin, so I've only got to define User and 
Bookmark. 

The model classes define validation rules for the database fields. If a client sends bad 
data (such as trying to create a user without specifying a name) , the appropriate vali- 
dation rule is triggered and the controller method sends the client a response code of 
400 ("Bad Request"). The same model classes could be used in a conventional web 
application, or a GUI application. The validation errors would be displayed differently, 
but the same rules would always apply. 

The model classes also define a few methods which work against the database. These 
methods are used by the controllers. 



Model Code | 205 



The User Model 



This is the simpler of the two models (see Example 7-24) . It has some validation rules, 
a one-to-many relationship with Bookmark objects, and a few methods (called by the 
controllers) for validating passwords. 

Example 7-24. app/models/user.rb 

class User < ActiveRecord: :Base 

# A user has many bookmarks. When the user is destroyed, 

# all their bookmarks should also be destroyed, 
hasjnany :bookmarks, dependent => :destroy 

# A user must have a unique username. 
validates_uniqueness_of :name 

# A user must have a username, full name, and email. 
validates_presence_of :name, :full_name, :email 

# Make sure passwords are never stored in plaintext, by running them 

# through a one-way hash as soon as possible, 
def password=(password) 

super(User. hashed (password)) 
end 

# Given a username and password, returns a User object if the 

# password matches the hashed one on file. Otherwise, returns nil. 
def self .authenticated_user(username, pass) 

user = find_by_name(username) 
if user 

user = nil unless hashed(pass) == user. password 
end 

return user 
end 

# Performs a one-way hash of some data, 
def self .hashed(password) 

Digest: :SHAl.new(password) .to_s 
end 
end 

The Bookmark Model 

This is a more complicated model (see Example 7-25). First, let's define the relation- 
ships between Bookmark and the other model classes, along with some validation rules 
and a rule for generating the MD5 hash of a URL We have to keep this information 
because the MD5 calculation only works in one direction. If a client requests /vl/uris/ 
55020a5384313579a5f Ile75cl8l8b89, we can't reverse the MD5 calculation. We need to 
be able to look up a URI by its MD5 hash. 



206 | Chapter 7: A Service Implementation 



Example 7-25. app/models/bookmark.rb 

class Bookmark < ActiveRecord: :Base 

# Every bookmark belongs to some user, 
belongsjto :user 

# A bookmark can have tags. The relationships between bookmarks and 

# tags are managed by the acts_as_taggable plugin. 
acts_as_taggable 

# A bookmark must have an associated user ID, a URI, a short 

# description, and a timestamp. 

validates_presence_of :user_id, :uri, :short_description, :timestamp 

# The URI hash should never be changed directly: only when the URI 

# changes. 

attr_protected :uri_hash 

# And., here's the code to update the URI hash when the URI changes, 
def uri=(new_uri) 

super 

self .uri_hash = Digest: :MD5.new(new_uri).to_s 
end 

# This method is triggered by Bookmark. new and by 

# Bookmark#update_attributes. It replaces a bookmark's current set 

# of tags with a new set. 
def tag_with(tags) 

Tag. transaction do 
taggings . destroy_all 

tags. each { | name | Tag.find_or_create_by_name(name) .on(self ) } 
end 
end 

That last method makes it possible to associate tags with bookmarks. The acts_as_tag 
gable plugin allows me to do basic queries like "what bookmarks are tagged with 'ru- 
by'?" Unfortunately, I usually need slightly more complex queries, like "what book- 
marks belonging to leonardr are tagged with 'ruby'?", so I can't use the plugin's 
find_tagged_with method. I need to define my own method that attaches a tag restric- 
tion to some preexisting restriction like "bookmarks belonging to leonardr." 

This custom_find method is the workhorse of the whole service, since it's called by the 
ApplicationController#show_bookmarks method, which is called by many of the REST- 
ful Rails actions (see Example 7-26). 

Example 7-26. app/models/bookmark.rb continued 

# This method finds bookmarks, possibly ones tagged with a 

# particular tag. 

def self .custom_find(conditions, tag=nil, limit=nil) 
if tag 

# When a tag restriction is specified, we have to find bookmarks 

# the hard way: by constructing a SQt query that matches only 

# bookmarks tagged with the right tag. 

sql = ["SEtECT bookmarks.* FROM bookmarks, tags, taggings" + 



Model Code | 207 



" WHERE taggings.taggable_type = 'Bookmark'" + 
" AND bookmarks. id = taggings.taggable_id" + 
" AND taggings.tag_id = tags. id AND tags. name = ?", 
tag] 
if conditions 
sql[o] << " AND " << conditions[o] 
sql += conditions[l. .conditions. size] 
end 

sql[o] « " ORDER BY bookmarks. timestamp DESC" 
sql[o] << " LIMIT " << limit. to_i.to_s if limit 
bookmarks = find_by_sql(sql) 
else 

# Without a tag restriction, we can find bookmarks the easy way: 

# with the superclass find() implementation. 

bookmarks = find(:all, {conditions => conditions, :limit => limit, 
:order => 'timestamp DESC'}) 

end 

return bookmarks 
end 

There are two more database-related methods (see Example 7-27). The 
Bookmark. only_visible_to! method manipulates a set of ActiveRecord conditions so 
that they only apply to bookmarks the given user can see. The Bookmark. calendar 
method groups a user's bookmarks by the date they were posted. This implementation 
may not work for you, since it uses a SQL function (DATE) that's not available for all 
databases. 

Example 7-27. app/models/bookmark.rb concluded 

# Restricts a bookmark query so that it only finds bookmarks visible 

# to the given user. This means public bookmarks, and the given 

# user's private bookmarks. 

def self .only_visible_to! (conditions, user) 

# The first element in the "conditions" array is a SQL WHERE 

# clause with variable substitutions. The subsequent elements are 

# the variables whose values will be substituted. For instance, 

# if "conditions" starts out empty: [""]... 

conditions[o] << " AND " unless conditions[o] .empty? 
conditions[o] << "(public='l' " 
if user 

conditions[o] << " OR user_id=?" 
conditions << user. id 
end 

conditions[o] << ")" 

# ...its value might now be ["(public='l' or user_id=?)", 55]. 

# ActiveRecord knows how to turn this into the SQL WHERE clause 

# "(public='l' or user_id=55)". 
end 

# This method retrieves data for the CalendarController. It uses the 

# SQL DATE() function to group together entries made on a particular 

# day. 



208 | Chapter 7: A Service Implementation 



def self .calendar(user_id, viewed_by_owner, tag=nil) 
if tag 

tag_from = ", tags, taggings" 

tag_where = "AND taggings. taggable_type = 'Bookmark'" + 
" AND bookmarks. id = taggings. taggable_id" + 
" AND taggings. tag_id = tags. id AND tags. name = ?" 

end 

# Unless a user is viewing their own calendar, only count public 

# bookmarks. 

public_where = viewed_by_owner ? "" : "AND public='l'" 

sql = ["SELECT date(timestamp) AS date, count(bookmarks.id) AS count" + 

" FROM bookmarks#{tag_from} " + 

" WHERE user_id=? #{tag_where} #{public_where} " + 

" GROUP BY date(timestamp)", user_id] 
sql << tag if tag 

# This will return a list of rather bizarre ActiveRecord objects, 

# which CalendarController knows how to turn into an XML document. 
find_by_sql(sql) 

end 
end 

Now you should be ready to start your Rails server in a console window, and start using 
the web service. 
$ script/server 

What Does the Client Need to Know? 

Of course, using the web service just means writing more code. Unlike a Rails service 
generated with script/generate scaffold_resource (see "Clients Made Transparent 
with ActiveResource" in Chapter 3), this service can't be used as a web site. I didn't 
create any HTML forms or HTML-based views of the data. This was done mainly for 
space reasons. Look back at Example 7-8 and the call to respond_to. It's got a call to 
format, xml and a call to format, atom, and so on. That's the sort of place I'd put a call 
to format.html, to render an ERb template as HTML. 

Eventually the site will be well-populated with peoples' bookmarks, and the site will 
expose many interesting resources as interlinked Atom representations. Any program, 
including today's web browsers, can take these resources as input: the client just needs 
to speak HTTP GET and know what to do with a syndication file. 

But how are those resources supposed to get on the site in the first place? The only 
existing general-purpose web service client is the web browser, and I haven't provided 
any HTML forms for creating users or posting bookmarks. Even if I did, that would 
only take care of situations where the client is under the direct control of a human being. 



What Does the Client Need to Know? | 209 



Natural-Language Service Description 

There are three possibilities for making it easy to write clients; they're more or less the 
ones I covered in Chapters 2 and 3. The simplest is to publish an English description 
of the service's layout. If someone wants to use my service they can study my description 
and write custom HTTP client code. 

Most of today's RESTful and hybrid web services work this way. Instead of specifying 
the levers of state in hypermedia, they specify the levers in regular media — English text 
— which a human must interpret ahead of time. You'll need a basic natural-language 
description of your service anyway, to serve as advertisement. You want people to 
immediately see what your service does and want to use it. 

I've already got a prose description of my social bookmarking service: it takes up much 
of this chapter. Example 7-28 is a simple command-line Ruby client for the service, 
based on that prose description. This client knows enough to create user accounts and 
post bookmarks. 

Example 7-28. A rest-open-uri client for the bookmark service 

#!/usr/bin/ruby 

#open -uri- bookmark-client .rb 

require 'rubygems' 

require 'rest-open-uri' 

require 'uri' 

require 'cgi' 

# An HTTP-based Ruby client for my social bookmarking service 
class BookmarkClient 

def initialize(service_root) 

@service_root = service_root 
end 

# Turn a Ruby hash into a form-encoded set of key- value pairs, 
def form_encoded(hash) 

encoded = [] 

hash. each do | key, value| 

encoded << CGI.escape(key) + '=' + CGI.escape(value) 
end 

return encoded. join( '&' ) 
end 

# Create a new user. 

def new_user(username, password, full_name, email) 
representation = form_encoded({ "user[name]" => username, 

"user[password] " => password, 
"user[full_name] " => full_name, 
"user[email]" => email }) 

puts representation 
begin 

response = open(@service_root + '/users', :method => :post, 
:body => representation) 



210 | Chapter 7: A Service Implementation 



puts "User #{username} created at #{response.meta[ ' location' ]}" 
rescue OpenllRI : : HTTPError => e 
response_code = e. io. status [o] .to_i 
if response_code == "409" # Conflict 

puts "Sorry, there's already a user called #{username}. " 
else 

raise e 
end 
end 
end 

# Post a new bookmark for the given user. 

def new_bookmark(username, password, uri, short_description) 
representation = form_encoded({ "bookmark[uri] " => uri, 

"bookmark[ short_description ] " => 
short_description }) 

begin 

dest = "#{@service_root}/users/#{URI.encode(username)}/bookmarks" 
response = open(dest, :method => :post, :body => representation, 

:http_basic_authentication => [username, password]) 
puts "Bookmark posted to #{response.meta[ 'location' ]}" 
rescue OpenURI: : HTTPError => e 
response_code = e. io. status [o] .to_i 
if response_code == 401 # Unauthorized 

puts "It looks like you gave me a bad password." 
elsif response_code == 409 # Conflict 

puts "It looks like you already posted that bookmark." 
else 

raise e 
end 
end 
end 
end 

# Main application 
command = ARGV. shift 

if ARGV. size != 4 | | (command != "new-user" && command != "new-bookmark") 
puts "Usage: #{$0} new-user [username] [password] [full name] [email]" 
puts "Usage: #{$0} new-bookmark [username] [password]" + 

" [URI] [short description]" 
exit 

end 

client = BookmarkClient.new('http://localhost:3000/vl') 

if command == "new-user" 
username, password, full_name, email = ARGV 
client. new_user(username, password, full_name, email) 

else 

username, password, uri, short_description = ARGV 
client. new_bookmark(username, password, uri, short_description) 
end 



What Does the Client Need to Know? | 211 



Description Through Standardization 

One alternative to explaining everything is to make your service like other services. If 
all services exposed the same representation formats, and mapped URIs to resources 
in the same way... well, we can't get rid of client programming altogether, but clients 
could work on a higher level than HTTP. 'Conventions are powerful tools: in fact, 
they're the same tools that REST uses. Every RESTful resource-oriented web service 
uses URIs to designate resources, and expresses operations in terms of HTTP's uniform 
interface. The idea here is to apply higher-level conventions than REST's, so that the 
client programmer doesn't have to write as much code. 

Take the Rails architecture as an example. Rails is good at gently imposing its design 
preferences on the programmer. The result is that most RESTful Rails services do the 
same kind of thing in the same way. At bottom, the job of almost every Rails service is 
to send and accept representations of ActiveRecord objects. These services all map URIs 
to Rails controllers, Rails controllers to resources, resources to ActiveRecord objects, 
and ActiveRecord objects to rows in the database. The representation formats are also 
standardized: either as XML documents like the one in Example 7-4, or form-encoded 
key-value pairs like the ones in Example 7-5. They're not the best representation for- 
mats, because it's difficult to make connected services out of them, but they're OK. 

The ActiveResource library, currently under development, is a client library that takes 
advantage of these similarities between Rails services. I first mentioned ActiveResource 
in Chapter 3, where I showed it in action against a very simple Rails service. It doesn't 
replace custom client code, but it hides the details of HTTP access behind an interface 
that looks like ActiveRecord. The ActiveResource/ActiveRecord approach won't work 
for all web services, or even all Rails web services. It doesn't work very well on this 
service. But it's not quite fair for me to judge ActiveResource by these standards, since 
it's still in development. As of the time of writing, it's more a promising possiblity than 
a real-world solution to a problem. 

Hypermedia Descriptions 

Even when the Ruby ActiveResource client is improved and officially released, it will 
be nothing more than the embodiment of some high-level design conventions. The 
conventions are useful: another web service framework might copy these conventions, 
and then Ruby's ActiveResource client would work with it. An ActiveResource library 
written in another language will work with Rails services. But if a service doesn't follow 
the conventions, ActiveResource can't talk to it. 

What we need is a general framework, a way for each individual service to tell the client 
about its resource design, its representation formats, and the links it provides between 



There will always be client-side code for translating the needs of the user into web service operations. The 
only exception is in a web browser, where the user is right there, guiding the client through every step. 



212 | Chapter 7: A Service Implementation 



resources. That will give us some of the benefits of standardized conventions, without 
forcing all web services to comply with more than a few minimal requirements. 

This brings us full circle to the REST notion of connectedness, of "hypermedia as the 
engine of application state." I talk about connectedness so much because hypermedia 
links and forms are these machine-readable conventions for describing the differences 
between services. If your service only serves serialized data structures that show the 
current resource state, then of course you start thinking about additional standards and 
conventions. Your representations are only doing half a job. 

We don't think the human web needs these additional standards, because the human 
web serves documents full of links and forms, not serialized data structures that need 
extra interpretation. The links and forms on the human web tell our web browsers how 
to manipulate application and resource state, in response to our expressed desires. It 
doesn't matter that every web site was designed by a different person, because the 
differences between them are represented in machine-readable format. 

The XHTML links and forms in Chapters 5 and 6 are machine-readable descriptions 
of what makes the fantasy map service different from other services. In this chapter, 
the links embedded in the Atom documents are machine-readable descriptions of the 
connections that distinguish this service from others that serve Atom documents. In 
Chapter 9 I'll consider three major hypermedia formats that can describe these differ- 
ences between services: XHTML 4, XHTML 5, and WADL. For now, though, it's time 
to take a step back and take a look at REST and the ROA as a whole. 



What Does the Client Need to Know? | 213 



CHAPTER 8 



REST and ROA Best Practices 



By now you should have a good idea of how to build resource-oriented, RESTful web 
services. This chapter is a pause to gather in one place the most important ideas so far, 
and to fill in some of the gaps in my coverage. 

The gaps exist because the theoretical chapters have focused on basics, and the practical 
chapters have worked with specific services. I've implemented conditional HTTP GET 
but I haven't explained it. I've implemented HTTP Basic authentication and a client 
for Amazon's custom authentication mechanism, but I haven't compared them to other 
kinds of HTTP authentication, and I've glossed over the problem of authenticating a 
client to its own user. 

The first part of this chapter is a recap of the main ideas of REST and the ROA. The 
second part describes the ideas I haven't already covered. I talk about specific features 
of HTTP and tough cases in resource design. In Chapter 9 1 discuss the building blocks 
of services: specific technologies and patterns that have been used to make successful 
web services. Taken together, this chapter and the next form a practical reference for 
RESTful web services. You can consult them as needed when making technology or 
design decisions. 

Resource-Oriented Basics 

The only differences between a web service and a web site are the audience (preprog- 
rammed clients instead of human beings) and a few client capabilities. Both web serv- 
ices and web sites benefit from a resource-oriented design based on HTTP, URIs, and 
(usually) XML. 

Every interesting thing your application manages should be exposed as a resource. A 
resource can be anything a client might want to link to: a work of art, a piece of infor- 
mation, a physical object, a concept, or a grouping of references to other resources. 

A URI is the name of a resource. Every resource must have at least one name. A resource 
should have as few names as possible, and every name should be meaningful. 



215 



The client cannot access resources directly. A web service serves representations of a 
resource: documents in specific data formats that contain information about the re- 
source. The difference between a resource and its representation is somewhat academic 
for static web sites, where the resources are just files on disk that are sent verbatim to 
clients. The distinction takes on greater importance when the resource is a row in a 
database, a physical object, an abstract concept, or a real-world event in progress. 

All access to resources happens through HTTP's uniform interface. These are the four 
basic HTTP verbs (GET, POST, PUT, and DELETE), and the two auxiliaries (HEAD 
and OPTIONS) . Put complexity in your representations, in the variety of resources you 
expose, and in the links between resources. Don't put it in the access methods. 

The Generic ROA Procedure 

Reprinted from Chapter 6, this is an all-purpose procedure for splitting a problem space 
into RESTful resources. 

This procedure only takes into account the constraints of REST and the ROA. Your 
choice of framework may impose additional constraints. If so, you might as well take 
those into account while you're designing the resources. In Chapter 12 1 give a modified 
version of this procedure that works with Ruby on Rails. 

1 . Figure out the data set 

2. Split the data set into resources 
For each kind of resource: 

3. Name the resources with URIs 

4. Expose a subset of the uniform interface 

5. Design the representation (s) accepted from the client 

6. Design the representation (s) served to the client 

7. Integrate this resource into existing resources, using hypermedia links and forms 

8. Consider the typical course of events: what's supposed to happen? Standard con- 
trol flows like the Atom Publishing Protocol can help (see Chapter 9). 

9. Consider error conditions: what might go wrong? Again, standard control flows 
can help. 

Addressability 

A web service is addressable if it exposes the interesting aspects of its data set through 
resources. Every resource has its own unique URI: in fact, URI just stands for "Universal 
Resource Identifier." Most RESTful web services expose an infinite number of URIs. 
Most RPC-style web services expose very few URIs, often as few as one. 



216 | Chapter 8: REST and ROA Best Practices 



Representations Should Be Addressable 

A URI should never represent more than one resource. Then it wouldn't be a Univer- 
sal Resource Identifier. Furthermore, I suggest that every representation of a resource 
should have its own URI. This is because URIs are often passed around or used as input 
to other web services. The expectation then is that the URI designates a particular 
representation of the resource. 

Let's say you've exposed a press release at /releases/104. There's an English and a 
Spanish version of the press release, an HTML and plain-text version of each. Your 
clients should be able set the Accept- Language request header to choose an English or 
Spanish representation of /releases/104, and the Accept request header to choose an 
HTML or plain-text representation. But you should also give each representation a 
separate URI: maybe URIs like /releases/104, en, /releases/104, es. html, 
and /releases/104. txt. 

When a client requests one of the representation-specific URIs, you should set the 
Content -Location response header to /releases/104. This lets the client know the can- 
onical location of the "press release" resource. If the client wants to talk about the press 
release independent of any particular language and format, it can link to that canonical 
URI. If it wants to talk about the press release in a particular language and/or format, 
the client can link to the URI it requested. 

In the bookmarking service from Chapter 7, 1 exposed two representations of a set of 
bookmarks: a generic XML representation at /vl/users/leonardr/bookmarks.xml, and 
an Atom representation at /vl/users/leonardr/bookmarks.atom. I also exposed a can- 
onical URI for the resource at /vl/users/leonardr/bookmarks. A client can set its 
Accept request header to distinguish between Atom and generic XML representations 
of /vl/users/leonardr/bookmarks, or it can tweak the URI to get a different represen- 
tation. Both techniques work, and both techniques are RESTful, but a URI travels better 
across clients if it specifies a resource and a representation. 

It's OK for a client to send information in HTTP request headers, so long as the server 
doesn't make that the only way of selecting a resource or representation. Headers can 
also contain sensitive information like authentication credentials, or information that's 
different for every client. But headers shouldn't be the only tool a client has to specify 
which representation is served or which resource is selected. 

State and Statelessness 

There are two types of state in a RESTful service. There's resource state, which is in- 
formation about resources, and application state, which is information about the path 
the client has taken through the application. Resource state stays on the server and is 
only sent to the client in the form of representations. Application state stays on the 
client until it can be used to create, modify, or delete a resource. Then it's sent to the 
server as part of a POST, PUT, or DELETE request, and becomes resource state. 



State and Statelessness | 217 



A RESTful service is "stateless" if the server never stores any application state. In a 
stateless application, the server considers each client request in isolation and in terms 
of the current resource state. If the client wants any application state to be taken into 
consideration, the client must submit it as part of the request. This includes things like 
authentication credentials, which are submitted with every request. 

The client manipulates resource state by sending a representation as part of a PUT or 
POST request. (DELETE requests work the same way, but there's no representation.) 
The server manipulates client state by sending representations in response to the client's 
GET requests. This is where the name "Representational State Transfer" comes from. 

Connectedness 

The server can guide the client from one application state to another by sending links 
and forms in its representations. I call this connectedness because the links and forms 
connect the resources to each other. The Fielding thesis calls this "hypermedia as the 
engine of application state." 

In a well-connected service, the client can make a path through the application by 
following links and filling out forms. In a service that's not connected, the client must 
use predefined rules to construct every URI it wants to visit. Right now the human web 
is very well-connected, because most pages on a web site can be reached by following 
links from the main page. Right now the programmable web is not very well-connected. 

The server can also guide the client from one resource state to another by sending forms 
in its representations. Forms guide the client through the process of modifying resource 
state with a PUT or POST request, by giving hints about what representations are 
acceptable. 

Links and forms reveal the levers of state: requests the client might make in the future 
to change application or resource state. Of course, the levers of state can be exposed 
only when the representation format supports links or forms. A hypermedia format like 
XHTML is good for this; so is an XML format that can have XHTML or WADL em- 
bedded in it. 

The Uniform Interface 

All interaction between clients and resources is mediated through a few basic HTTP 
methods. Any resource will expose some or all of these methods, and a method does 
the same thing on every resource that supports it. 

A GET request is a request for information about a resource. The information is deliv- 
ered as a set of headers and a representation. The client never sends a representation 
along with a GET request. 



218 | Chapter 8: REST and ROA Best Practices 



A HEAD request is the same as a GET request, except that only the headers are sent in 
response. The representation is omitted. 

A PUT request is an assertion about the state of a resource. The client usually sends a 
representation along with a PUT request, and the server tries to create or change the 
resource so that its state matches what the representation says. A PUT request with no 
representation is just an assertion that a resource should exist at a certain URI. 

A DELETE request is an assertion that a resource should no longer exist. The client 
never sends a representation along with a DELETE request. 

A POST request is an attempt to create a new resource from an existing one. The existing 
resource may be the parent of the new one in a data-structure sense, the way the root 
of a tree is the parent of all its leaf nodes. Or the existing resource may be a special 
"factory" resource whose only purpose is to generate other resources. The representa- 
tion sent along with a POST request describes the initial state of the new resource. As 
with PUT, a POST request doesn't need to include a representation at all. 

A POST request may also be used to append to the state of an existing resource, without 
creating a whole new resource. 

An OPTIONS request is an attempt to discover the levers of state: to find out which 
subset of the uniform interface a resource supports. It's rarely used. Today's services 
specify the levers of state up front, either in human-readable documentation or in hy- 
permedia documents like XHTML and WADL files. 

If you find yourself wanting to add another method or additional features to HTTP, 
you can overload POST (see "Overloading POST), but you probably need to add an- 
other kind of resource. If you start wanting to add transactional support to HTTP, you 
should probably expose transactions as resources that can be created, updated, and 
deleted. See "Resource Design" later in this chapter for more on this technique. 

Safety and Idempotence 

A GET or HEAD request should be safe: a client that makes a GET or HEAD request 
is not requesting any changes to server state. The server might decide on its own to 
change state (maybe by logging the request or incrementing a hit counter), but it should 
not hold the client responsible for those changes. Making any number of GET requests 
to a certain URI should have the same practical effect as making no requests at all. 

A PUT or DELETE request should be idempotent. Making more than one PUT or DE- 
LETE request to a given URI should have the same effect as making only one. One 
common problem: PUT requests that set resource state in relative terms like "increment 
value by 5." Making 10 PUT requests like that is a lot different from just making one. 
PUT requests should set items of resource state to specific values. 

The safe methods, GET and HEAD, are automatically idempotent as well. POST re- 
quests for resource creation are neither safe nor idempotent. An overloaded POST 



The Uniform Interface | 219 



request might or might not be safe or idempotent. There's no way for a client to tell, 
since overloaded POST can do anything at all. You can make POST idempotent with 
POST Once Exactly (see Chapter 9). 

New Resources: PUT Versus POST 

You can expose the creation of new resources through PUT, POST, or both. But a client 
can only use PUT to create resources when it can calculate the final URI of the new 
resource. In Amazon's S3 service, the URI path to a bucket is /{bucket-name}. Since the 
client chooses the bucket name, a client can create a bucket by constructing the cor- 
responding URI and sending a PUT request to it. 

On the other hand, the URI to a resource in a typical Rails web service looks 
like /{database-table-name}/{database-ID}. The name of the database table is known 
in advance, but the ID of the new resource won't be known until the corresponding 
record is saved to the database. To create a resource, the client must POST to a "factory" 
resource, located at /{database-table-name}. The server chooses a URI for the new 
resource. 

Overloading POST 

POST isn't just for creating new resources and appending to representations. You can 
also use it to turn a resource into a tiny RPC-style message processor. A resource that 
receives an overloaded POST request can scan the incoming representation for addi- 
tional method information, and carry out any task whatsoever. This gives the resource 
a wider vocabulary than one that supports only the uniform interface. 

This is how most web applications work. XML-RPC and SOAP/WSDL web services 
also run over overloaded POST. I strongly discourage the use of overloaded POST, 
because it ruins the uniform interface. If you're tempted to expose complex objects or 
processes through overloaded POST, try giving the objects or processes their own URIs, 
and exposing them as resources. I show several examples of this in "Resource De- 
sign" later in this chapter. 

There are two noncontroversial uses for overloaded POST. The first is to simulate 
HTTP's uniform interface for clients like web browsers that don't support PUT or 
DELETE. The second is to work around limits on the maximum length of a URI. The 
HTTP standard specifies no limit on how long a URI can get, but many clients and 
servers impose their own limits: Apache won't respond to requests for URIs longer than 
8 KB. If a client can't make a GET request to http://www.example.com/numbers/ 
1111111 because of URI length restrictions (imagine a million more ones there if you 
like), it can make a POST request to http://www.example.com/numbersP_meth- 
od=GET and put "1111111" in the entity-body. 

If you want to do without PUT and DELETE altogether, it's entirely RESTful to expose 
safe operations on resources through GET, and all other operations through overloaded 



220 | Chapter 8: REST and ROA Best Practices 



POST. Doing this violates my Resource-Oriented Architecture, but it conforms to the 
less restrictive rules of REST. REST says you should use a uniform interface, but it 
doesn't say which one. 

If the uniform interface really doesn't work for you, or it's not worth the effort to make 
it work, then go ahead and overload POST, but don't lose the resource-oriented design. 
Every URI you expose should still be a resource: something a client might want to link 
to. A lot of web applications create new URIs for operations exposed through overloa- 
ded POST. You get URIs like /weblog/myweblog/rebuild-index. It doesn't make sense 
to link to that URI. Instead of putting method information in the URI, expose over- 
loaded POST on your existing resources (/weblog/myweblog) and ask for method infor- 
mation in the incoming representation (method=rebuild-index). This 
way, /weblog/myweblog still acts like a resource, albeit one that doesn't totally conform 
to the uniform interface. It responds to GET, PUT, DELETE... and also "rebuild-index" 
through overloaded POST. It's still an object in the object-oriented sense. 

A rule of thumb: if you're using overloaded POST, and you never expose GET and 
POST on the same URI, you're probably not exposing resources at all. You've probably 
got an RPC-style service. 

This Stuff Matters 

The principles of REST and the ROA are not arbitrary restrictions. They're simplifying 
assumptions that give advantages to resource-oriented services over the competition. 
RESTful resource-oriented services are simpler, easier to use, more interoperable, and 
easier to combine than RPC-style services. As I introduced the principles of the ROA 
in Chapter 4, 1 gave brief explanations of the ideas underlying the principles . In addition 
to recapping these ideas to help this chapter serve as a summary, I'd like to revisit them 
now in light of the real designs I've shown for resource-oriented services: the map 
service of Chapters 5 and 6, and the social bookmarking service of Chapter 7. 

Why Addressability Matters 

Addressability means that every interesting aspect of your service is immediately ac- 
cessible from outside. Every interesting aspect of your service has a URI: a unique 
identifier in a format that's familiar to every computer-literate person. This identifier 
can be bookmarked, passed around between applications, and used as a stand-in for 
the actual resource. Addressability makes it possible for others to make mashups of 
your service: to use it in ways you never imagined. 

In Chapter 4 I compared URIs to cell addresses in a spreadsheet, and to file paths in a 
command-line shell. The web is powerful in the same way that spreadsheets and com- 
mand-line shells are powerful. Every piece of information has a structured name that 
can be used as a reference to the real thing. 



This Stuff Matters | 221 



Why Statelessness Matters 

Statelessness is the simplifying assumption to beat all simplifying assumptions. Each 
of a client's requests contains all application states necessary to understand that re- 
quest. None of this information is kept on the server, and none of it is implied by 
previous requests. Every request is handled in isolation and evaluated against the cur- 
rent resource state. 

This makes it trivial to scale your application up. If one server can't handle all the 
requests, just set up a load balancer and make a second server handle half the requests. 
Which half? It doesn't matter, because every request is self-contained. You can assign 
requests to servers randomly, or with a simple round-robin algorithm. If two servers 
can't handle all the requests, you add a third server, ad infinitum. If one server goes 
down, the others automatically take over for it. When your application is stateless, you 
don't need to coordinate activity between servers, sharing memory or creating "server 
affinity" to make sure the same server handles every request in a "session." You can 
throw web servers at the problem until the bottleneck becomes access to your re- 
source state. Then you have to get into database replication, mirroring, or whatever 
strategy is most appropriate for the way you've chosen to store your resource state. 

Stateless applications are also more reliable. If a client makes a request that times out, 
statelessness means the client can resend the request without worrying that its "session" 
has gone into a strange state that it can't recover from. If it was a POST request, the 
client might have to worry about what the request did to the resource state, but that's 
a different story. The client has complete control over the application state at all times. 

There's an old joke. Patient: "Doctor, it hurts when I try to scale a system that keeps 
client state on the server!" Doctor: "Then don't do that." That's the idea behind state- 
lessness: don't do the thing that causes the trouble. 

Why the Uniform Interface Matters 

I covered this in detail near the end of Chapter 4, so I'll just give a brief recap here. If 
you say to me, "I've exposed a resource at http://www.example.com/myresource," that 
gives me no information about what that resource is, but it tells me a whole lot about 
how I can manipulate it. I know how to fetch a representation of it (GET), I know how 
to delete it (DELETE), I know roughly how to modify its state (PUT), and I know 
roughly how to spawn a subordinate resource from it (POST). 

There are still details to work out: which of these activities the resource actually sup- 
ports, 'which representation formats the resource serves and expects, and what this 
resource represents in the real world. But every resource works basically the same way 
and can be accessed with a universal client. This is a big part of the success of the Web. 

In theory, I know how to find out which of these activities are supported: send an OPTIONS request. But 
right now, nobody supports OPTIONS. 



222 | Chapter 8: REST and ROA Best Practices 




The conceptual links between S3 
resources 



Figure 8-1. We see links, but there are none 




Object 1.2 



Object 2.1 




Bucket list 



] 



The actual links, as revealed in the 
representations 



The restrictions imposed by the uniform interface (safety for GET and HEAD, idem- 
potence for PUT and DELETE), make HTTP more reliable. If your request didn't go 
through, you can keep resending it with no ill effects. The only exception is with POST 
requests. (See "POST Once Exactly" in Chapter 9 for ways of making POST idempo- 
tent.) 

The power of the uniform interface is not in the specific methods exposed. The human 
web has a different uniform interface — it uses GET for safe operations, and POST for 
everything else — and it does just fine. The power is the uniformity: everyone uses the 
same methods for everything. If you deviate from the ROA's uniform interface (say, by 
adopting the human web's uniform interface, or WebDAV's uniform interface), you 
switch communities: you gain compatibility with certain web services at the expense 
of others. 



Why Connectedness Matters 

Imagine the aggravation if instead of hypertext links, web pages gave you English in- 
structions on how to construct the URI to the next page. That's how most of today's 
RESTful web services work: the resources aren't connected to each other. This makes 
web services more brittle than human-oriented web sites, and it means that emergent 
properties of the Web (like Google's PageRank) don't happen on the programmable 
web. 

Look at Amazon S3. It's a perfectly respectable resource-oriented service. It's address- 
able, it's stateless, and it respects the uniform interface. But it's not connected at all. 
The representation of the S3 bucket list gives the name of each bucket, but it doesn't 
link to the buckets. The representation of a bucket gives the name of each object in the 
bucket, but it doesn't link to the objects. We humans know these objects are concep- 
tually linked, but there are no actual links in the representations (see Figure 8-1). 



This Stuff Matters | 223 



An S3 client can't get from one resource to another by following links. Instead it must 
internalize rules about how to construct the URI to a given bucket or object. These 
rules are given in the S3 technical documentation, not anywhere in the service itself. I 
demonstrated the rules in "Resources" in Chapter 3 . This wouldn't work on the human 
web, but in a web service we don't complain. Why is that? 

In general, we expect less from web services than from the human web. We experience 
the programmable web through customized clients, not generic clients like web brows- 
ers. These customized clients can be programmed with rules for URI construction. Most 
information on the programmable web is also available on the human web, so a lack 
of connectedness doesn't hide data from generic clients like search engines. Or else the 
information is hidden behind an authentication barrier and you don't want a search 
engine seeing it anyway. 

The S3 service gets away with a lack of connectedness because it only has three simple 
rules for URI construction. The URI to a bucket is just a slash and the URI-escaped 
name of the bucket. It's not difficult to program these rules into a client. The only bug 
that's at all likely is a failure to URI-escape the bucket or object name. Of course, there 
are additional rules for filtering and paginating the contents of buckets, which I skim- 
med over in Chapter 3. Those rules are more complex, and it would be better for S3 
representations to provide hypermedia forms instead of making clients construct these 
URIs on their own. 

More importantly, the S3 resources have simple and stable relationships to each other. 
The bucket list contains buckets, and a bucket contains objects. A link is just an indi- 
cation of a relationship between two resources. A simple relationship is easy to program 
into a client, and "contains" is one of the simplest. If a client is preprogrammed with 
the relationships between resources, links that only serve to convey those relationships 
are redundant. 

The social bookmarking service I implemented in Chapter 7 is a little better-connected 
than S3 . It represents lists of bookmarks as Atom documents full of internal and external 
links. But it's not totally connected: its representation of a user doesn't link to that 
user's bookmarks, posting history, or tag vocabulary (look back to Figure 7-1). And 
there's no information about where to find a user in the service, or how post a book- 
mark. The client is just supposed to know how to turn a username into a URI, and just 
supposed to know how to represent a bookmark. 

It's easy to see how this is theoretically unsatisfying. A service ought to be self-describ- 
ing, and not rely on some auxiliary English text that tells programmers how to write 
clients. It's also easy to see that a client that relies on rules for URI construction is more 
brittle. If the server changes those rules, it breaks all the clients. It's less easy to see the 
problems that stem from a lack of connectedness when the relationships between re- 
sources are complex or unstable. These problems can break clients even when the rules 
for URI construction never change. 



224 | Chapter 8: REST and ROA Best Practices 



Let's go back to the mapping service from Chapter 5. My representations were full of 
hyperlinks and forms, most of which were not technically necessary. Take this bit of 
markup from the representation of a road map that was in Example 5-6: 

<a class="zoom_in" href="/road.l/Earth/37.0; -95.8" />Zoom out</a> 
<a class="zoom_out" href="/road.3/Earth/37.0;-95.8" />Zoom in</a> 

Instead of providing these links everywhere, the service provider could put up an Eng- 
lish document telling the authors of automated clients how to manipulate the zoom 
level in the first path variable. That would disconnect some related resources (the road 
map at different zoom levels) , but it would save some bandwidth in every representation 
and it would have little effect on the actual code of any automated client. Personally, 
if I was writing a client for this service, I'd rather get from zoom level 8 to zoom level 
4 by setting road . 4 directly, than by following the "Zoom out" link over and over again. 
My client will break if the URI construction rule ever changes, but maybe I'm willing 
to take that risk. 

Now consider this bit of markup from the representation of the planet Earth. It's re- 
printed from Example 5-7: 

<dl class="place"> 
<dt>name</dt> <dd>Earth</dd> 
<dt>maps</dt> 
<ul class="maps"> 

<lixa class="map" href="/road/Earth">Road</ax/li> 

<lixa class="map" href="/satellite/Earth">Satellite</ax/li> 

</ul> 

The URIs are technically redundant. The name of the place indicates that these are 
maps of Earth, and the link text indicate that there's a satellite and a road map. Given 
those two pieces of information, a client can construct the corresponding map URI 
using a rule like the one for S3 objects: slash, map type, slash, planet name. Since the 
URIs can be replaced by a simple rule, the service might follow the S3 model and save 
some bandwidth by presenting the representation of Earth in an XML format like this: 

<place name="Earth" type="planet"> 
<map type="satellite" /> 
<map type="road" /> 

</place> 

If I was writing a client for this service, I would rather be given those links than have 
to construct them myself, but it's up for debate. 

Here's another bit of markup from Example 5-6. These links are to help the client move 
from one tile on the map to another. 

<a class="map_nav" href="46.05l8,-95.8">North</a> 

<a class="map_nav" href="4l.3776, -89.7698">Northeast</a> 

<a class="map_nav" href="36. 4642,-84. 5l87">East</a> 

<a class="map_nav" href="32. 3513,-90. 4459">Southeast</a> 



This Stuff Matters | 225 



It's technically possible for a client to generate these URIs based on rules. After all, the 
server is generating them based on rules. But the rules involve knowing how latitude 
and longitude work, the scale of the map at the current zoom level, and the size and 
shape of the planet. Any client programmer would agree it's easier to navigate a map 
by following the links than by calculating the coordinates of tiles. We've reached a point 
at which the relationships between resources are too complex to be expressed in simple 
rules. Connectedness becomes very important. 

This is where Google Maps's tile-based navigation system pays off (I described that 
system back in "Representing Maps and Points on Maps" in Chapter 5, if you're curi- 
ous) . Google Maps addresses its tiles by arbitrary X and Y coordinates instead of latitude 
and longitude. Finding the tile to the north is usually as easy as subtracting one from 
the value of Y. The relationships between tiles are much simpler. Nobody made me 
design my tile system in terms of latitude and longitude. If latitude/longitude calcula- 
tions are why I have to send navigation links along with every map representation, 
maybe I should rethink my strategy and expose simpler URIs, so that my clients can 
generate them more easily. 

But there's another reason why connectedness is valuable: it makes it possible for the 
client to handle relationships that change over time. Links not only hide the rules about 
how to build a URI for a given resource, they embody the rules of how resources are 
related to each other. Here's a terrifying example to illustrate the point. 

A terrifying example 

Suppose I get some new map data for my service. It's more accurate than the old data, 
but the scale is a little different. At zoom level 8, the client sees a slightly smaller map 
than it did before. Let's say at zoom level 8, a tile 256 pixels square now depicts an area 
three-quarters of a mile square, instead of seven-eigths of a mile square. 

At first glance, this has no effect on anything. Latitude and longitude haven't changed, 
so every point on the old map is in the same place on the new map. Google Maps-style 
tile URIs would break at this point, because they use X and Y instead of latitude and 
longitude. When the map data was updated, I'd have to recalculate all the tile images. 
Many points on the map would suddenly shift to different tiles, and get different X and 
Y coordinates. But all of my URIs still work. Every point on the map has the same URI 
it did before. 

In this new data set, the URI /road.8/Eartri/40.76;-73.98.png still shows part of the 
island of Manhattan, and the URI /road.8/Earth/40.7709,-73.98 still shows a point 
slightly to the north. But the rules have changed for finding the tile directly to the north 
of another tile. Those two tile graphics are centered on the same coordinates as before, 
but now each tile depicts a slightly smaller space. They used to be adjacent on the map, 
but now there's a gap between them (see Figure 8-2). 



226 | Chapter 8: REST and ROA Best Practices 




Figure 8-2. When clients choose URls for map tiles: before and after 

If a client application finds nearby tiles by following the navigation links I provide, it 
will automatically adapt to the new map scale. But an application that "already knows" 
how to turn latitude and longitude into image URIs will suddenly start showing maps 
that look like MAD Magazine fold-ins. 

I made a reasonable change to my service that didn't change any URIs, but it broke 
clients that always construct their own URIs. What changed was not the resources but 
the relationships between them: not the rules for constructing URIs but the rules for 
driving the application from one state to another. Those rules are embedded in my 
navigation links, and a client duplicates those rules at its own peril. 

And that's why it's important to connect your resources to each other. It's fine to expect 
your clients to use your rules to construct an initial URI (say, a certain place on the map 
at a certain zoom level), but if they need to navigate from one URI to another, you 
should provide appropriate links. As the programmable web matures, connectedness 
will become more and more important. 

Resource Design 

You'll need one resource for each "thing" exposed by your service. "Resource" is about 
as vague as "thing," so any kind of data or algorithm you want to expose can be a 
resource. There are three kinds of resources: 



Resource Design | 227 



• Predefined one-off resources, such as your service's home page or a static list of 
links to resources. A resource of this type corresponds to something you've only 
got a few of: maybe a class in an object-oriented system, or a database table in a 
database-oriented system. 

• A large (possibly infinite) number of resources corresponding to individual items 
of data. A resource of this type might correspond to an object in an object-oriented 
system, or a database row in a database-oriented system. 

• A large (probably infinite) number of resources corresponding to the possible out- 
puts of an algorithm. A resource of this type might correspond to the results of a 
query in a database-oriented system. Lists of search results and filtered lists of 
resources fall into this category. 

There are some difficult cases in resource design, places where it seems you must ma- 
nipulate a resource in a way that doesn't fit the uniform interface. The answer is almost 
always to expose the thing that's causing the problem as a new set of resources. These 
new resources may be more abstract then the rest of your resources, but that's fine: a 
resource can be anything. 

Relationships Between Resources 

Suppose Alice and Bob are resources in my service. That is, they're people in the real 
world, but my service gives them URIs and offers representations of their state. One 
day Alice and Bob get married. How should this be represented in my service? 

A client can PUT to Alice's URI, modifying her state to reflect the fact that she's married 
to Bob, and then PUT to Bob's URI to say he's married to Alice. That's not very satis- 
fying because it's two steps. A client might PUT to Alice's URI and forget to PUT to 
Bob's. Now Alice is married to Bob but not vice versa. 

Instead I should treat the marriage, this relationship between two resources, as a thing 
in itself: a third resource. A client can declare two people married by sending a PUT 
request to a "marriage" URI or a POST request to a "registrar" URI (it depends on how 
I choose to do the design). The representation includes links to Alice and Bob's URIs: 
it's an assertion that the two are married. The server applies any appropriate rules about 
who's allowed to get married, and either sends an error message or creates a new re- 
source representing the marriage. Other resources can now link to this resource, and 
it responds to the uniform interface. A client can GET it or DELETE it (though hopefully 
DELETEing it won't be necessary). 

Asynchronous Operations 

HTTP has a synchronous request-response model. The client opens an Internet socket 
to the server, makes its request, and keeps the socket open until the server has sent the 
response. If the client doesn't care about the response it can close the socket early, but 
to get a response it must leave the socket open until the server is ready. 



228 | Chapter 8: REST and ROA Best Practices 



The problem is not all operations can be completed in the time we expect an HTTP 
request to take. Some operations take hours or days. An HTTP request would surely 
be timed out after that kind of inactivity. Even if it didn't, who wants to keep a socket 
open for days just waiting for a server to respond? Is there no way to expose such 
operations asynchronously through HTTP? 

There is, but it requires that the operation be split into two or more synchronous re- 
quests. The first request spawns the operation, and subsequent requests let the client 
learn about the status of the operation. The secret is the status code 202 ("Accepted"). 

I'll demonstrate one strategy for implementing asynchronous requests with the 202 
status code. Let's say we have a web service that handles a queue of requests. The client 
makes its service request normally, possibly without any knowledge that the request 
will be handled asynchronously. It sends a request like this one: 

POST /queue HTTP/l.l 
Host: jobs.example.com 

Authorization: Basic m0lTcm4hbAr3gBUzv3kcceP= 

Give me the prime factorization of this 100000 -digit number: 

The server accepts the request, creates a new job, and puts it at the end of the queue. 
It will take a long time for the new job to be completed, or there wouldn't be a need 
for a queue in the first place. Instead of keeping the client waiting until the job finally 
runs, the server sends this response right away: 

202 Accepted 

Location : http : //jobs . example . com/queue/joblla4f 9 

The server has created a new "job" resource and given it a URI that doesn't conflict 
with any other job. The asynchronous operation is now in progress, and the client can 
make GET requests to that URI to see how it's going — that is, to get the current state 
of the "job" resource. Once the operation is complete, any results will become available 
as a representation of this resource. Once the client is done reading the results it can 
DELETE the job resource. The client may even be able to cancel the operation by 
DELETEing its job prematurely. 

Again, I've overcome a perceived limitation of the Resource-Oriented Architecture by 
exposing a new kind of resource corresponding to the thing that was causing the prob- 
lem. In this case, the problem was how to handle asynchronous operations, and the 
solution was to expose each asynchronous operation as a new resource. 

There's one wrinkle. Because every request to start an asynchronous operation makes 
the server create a new resource (if only a transient one), such requests are neither safe 
nor idempotent. This means you can't spawn asynchronous operations with GET, 
DELETE, or (usually) PUT. The only HTTP method you can use and still respect the 
uniform interface is POST. This means you'll need to expose different resources for 
asynchronous operations than you would for synchronous operations. You'll probably 
do something like the job queue I just demonstrated. You'll expose a single resource 



Resource Design | 229 



— the job queue — to which the client POSTs to create a subordinate resource — the job. 
This will hold true whether the purpose of the asynchronous operation is to read some 
data, to make a calculation (as in the factoring example), or to modify the data set. 

Batch Operations 

Sometimes clients need to operate on more than one resource at once. You've already 
seen this: a list of search results is a kind of batch GET. Instead of fetching a set of 
resources one at a time, the client specifies some criteria and gets back a document 
containing abbreviated representations of many resources. I've also mentioned "fac- 
tory" resources that respond to POST and create subordinate resources. The factory 
idea is easy to scale up. If your clients need to create resources in bulk, you can expose 
a factory resource whose incoming representation describes a set of resources instead 
of just one, and creates many resources in response to a single request. 

What about modifying or deleting a set of resources at once? Existing resources are 
identified by URI, but addressability means an HTTP request can only point to a single 
URI, so how can you DELETE two resources at once? Remember that URIs can contain 
embedded URI paths, or even whole other URIs (if you escape them) . One way to let 
a client modify multiple resources at once is to expose a resource for every set of re- 
sources. For instance, http://www. example. com/sets/resourcel;subdir/resource2 might 
refer to a set of two resources: the one at http://www.example.com/resourcel and the 
one at http://www.example.com/subdir/resource2. Send a DELETE to that "set" re- 
source and you delete both resources in the set. Send a PUT instead, with a represen- 
tation of each resource in the set, and you can modify both resources with a single 
HTTP request. 

You might be wondering what HTTP status code to send in response to a batch oper- 
ation. After all, one of those PUTs might succeed while the other one fails. Should the 
status code be 200 ("OK") or 500 ("Internal Server Error")? One solution is to make a 
batch operation spawn a series of asynchronous jobs. Then you can send 202 ("Ac- 
cepted"), and show the client how to check on the status of the individual jobs. Or, 
you can use an extended HTTP status code created by the WebDAV extension to 
HTTP: 207 ("Multi-Status"). 

The 207 status code tells the client to look in the entity-body for a list of status codes 
like 200 ("OK") and 500 ("Internal Server Error"). The entity-body is an XML docu- 
ment that tells the client which operations succeeded and which failed. This is not an 
ideal solution, since it moves information about what happened out of the status code 
and into the response entity-body. It's similar to the way overloaded POST moves the 
method information out of the HTTP method and into the request entity-body. But 
since there might be a different status code for every operation in the batch, you're 
really limited in your options here. Appendix B has more information about the 207 
status code. 



230 | Chapter 8: REST and R0A Best Practices 



Transactions 

In the Resource-Oriented Architecture, every incoming HTTP request has some re- 
source as its destination. But some services expose operations that span multiple re- 
sources. The classic example is an operation that transfers money from a checking to 
a savings account. In a database-backed system you'd use a transaction to prevent the 
possibility of losing or duplicating money. Is there a resource-oriented way to imple- 
ment transactions? 

You can expose simple transactions as batch operations, or use overloaded POST, but 
here's another way. It involves (you guessed it) exposing the transactions themselves 
as resources. I'll show you a sample transaction using the account transfer example. 
Let's say the "checking account" resource is exposed at /accounts/checking/ll, and 
the "savings account" resource is exposed at /accounts/ savings/55 . Both accounts have 
a current balance of $200, and I want to transfer $50 from checking to savings. 

I'll quickly walk you through the requests and then explain them. First I create a trans- 
action by sending a POST to a transaction factory resource: 

POST /transactions/account-transfer HTTP/l.l 
Host: example.com 

The response gives me the URI of my newly created transaction resource: 
201 Created 

Location : /transactions/account-transfer/lla5 

I PUT the first part of my transaction: the new, reduced balance of the checking account. 

PUT /transact ions/account -transfer/lla5/accounts/checking/ll HTTP/l.l 
Host: example.com 

balance=150 

I PUT the second part of my transaction: the new, increased balance of the savings 
account. 

PUT /transactions/account-transfer/lla5/accounts/savings/55 HTTP/l.l 
Host: example.com 

balance=250 

At any point up to this I can DELETE the transaction resource to roll back the trans- 
action. Instead, I'm going to commit the transaction: 

PUT /transactions/account-transfer/llas HTTP/l.l 
Host: example.com 

committed=true 

This is the server's chance to make sure that the transaction doesn't create any incon- 
sistencies in resource state. For an "account transfer" transaction the server should 
check whether the transaction tries to create or destroy any money, or whether it tries 



Resource Design | 231 



to move money from one person to another without authorization. If everything checks 
out, here's the response I might get from my final PUT: 
200 OK 

Content-Type: application/xhtml+xml 

<a href="/accounts/checking/ll">Checking #ll</a>: New balance $150 
<a href="/accounts/savings/55">Savings #55</a>: New balance $250 

At this point I can DELETE the transaction and it won't be rolled back. Or the server 
might delete it automatically. More likely, it will be archived permanently as part of an 
audit trail. It's an addressable resource. Other resources, such as a list of transactions 
that affected checking account #11, can link to it. 

The challenge in representing transactions RESTfully is that every HTTP request is 
supposed to be a self-contained operation that operates on one resource. If you PUT a 
new balance to / accounts/ checking/ 11, then either the PUT succeeds or it doesn't. But 
during a transaction, the state of a resource is in flux. Look at the checking account 
from inside the transaction, and the balance is $150. Look at it from outside, and the 
balance is still $200. It's almost as though there are two different resources. 

That's how this solution presents it: as two different resources. There's the actual 
checking account, at /accounts/checking/ll, and there's one transaction's view of the 
checking account, at /transactions/account-transfer/lla5/accounts/checking/ll. 
When I PO$Ted to create /transactions/account-transfer/lla5/, the service exposed 
additional resources beneath the transaction URI: probably one resource for each ac- 
count on the system. I manipulated those resources as I would the corresponding ac- 
count resources, but my changes to resource state didn't go "live" until I committed 
the transaction. 

How would this be implemented behind the scenes? Probably with something that 
takes incoming requests and builds a queue of actions associated with the transaction. 
When the transaction is committed the server might start a database transaction, apply 
the queued actions, and then try to commit the database transaction. A failure to com- 
mit would be propagated as a failure to commit the web transaction. 

A RE$Tful transaction is more complex to implement than a database or programming 
language transaction. Every step in the transaction comes in as a separate HTTP re- 
quest. Every step identifies a resource and fits the uniform interface. It might be easier 
to punt and use overloaded POST. But if you implement transactions RESTfully, your 
transactions have the benefits of resources: they're addressable, operations on them are 
transparent, and they can be archived or linked to later. Yet again, the way to deal with 
an action that doesn't fit the uniform interface is to expose the action itself as a resource. 



232 | Chapter 8: REST and ROA Best Practices 



When In Doubt, Make It a Resource 

The techniques I've shown you are not the official RESTful or resource-oriented ways 
to handle transactions, asynchronous operations, and so on. They're just the best ones 
I could think up. If they don't work for you, you're free to try another arrangement. 

The larger point of this section is that when I say "anything can be a resource" I do 
mean anything. If there's a concept that's causing you design troubles, you can usually 
fit it into the ROA by exposing it as a new kind of resource. If you need to violate the 
uniform interface for performance reasons, you've always got overloaded POST. But 
just about anything can be made to respond to the uniform interface. 

URI Design 

URIs should be meaningful and well structured. Wherever possible, a client should be 
able to construct the URI for the resource they want to access. This increases the "sur- 
face area" of your application. It makes it possible for clients to get directly to any state 
of your application without having to traverse a bunch of intermediate resources. (But 
see "Why Connectedness Matters" earlier in this chapter; links are the most reliable 
way to convey the relationships between resources.) 

When designing URIs, use path variables to separate elements of a hierarchy, or a path 
through a directed graph. Example: /weblogs/myweblog/entries/100 goes from the 
general to the specific. From a list of weblogs, to a particular weblog, to the entries in 
that weblog, to a particular entry. Each path variable is in some sense "inside" the 
previous one. 

Use punctuation characters to separate multiple pieces of data at the same level of a 
hierarchy. Use commas when the order of the items matters, as it does in latitude and 
longitude: /Earth/37. 0,-95. 2. Use semicolons when the order doesn't matter: /color - 
blends/redjblue. 

Use query variables only to suggest arguments being plugged into an algorithm, or when 
the other two techniques fail. If two URIs differ only in their query variables, it implies 
that they're the different sets of inputs into the same underlying algorithm. 

URIs are supposed to designate resources, not operations on the resources. This means 
it's almost never appropriate to put the names of operations in your URIs. If you have 
a URI that looks like /object/do-operation, you're in danger of slipping into the RPC 
style. Nobody wants to link to do-operation: they want to link to the object. Expose 
the operation through the uniform interface, or use overloaded POST if you have to, 
but make your URIs designate objects, not operations on the objects. 

I can't make this an ironclad rule, because a resource can be anything. Operations on 
objects can be first-class objects, similar to how methods in a dynamic programming 
language are first-class objects, /object/do-operation might be a full-fledged resource 
that responds to GET, PUT, and DELETE. But if you're doing this, you're well ahead 



URI Design | 233 



of the current web services curve, and you've got weightier issues on your mind than 
whether you're contravening some best practice I set down in a book. 

Outgoing Representations 

Most of the documents you serve will be representations of resources, but some of them 
will be error conditions. Use HTTP status codes to convey how the client should regard 
the document you serve. If there's an error, you should set the status code to indicate 
an appropriate error condition, possibly 400 ("Bad Request"). Otherwise, the client 
might treat your error message as a representation of the resource it requested. 

The status code says what the document is for. The Content -Type response header says 
what format the document is in. Without this header, your clients won't know how to 
parse or handle the documents you serve. 

Representations should be human-readable, but computer-oriented. The job of the 
human web is to present information for direct human consumption. The main job of 
the programmable web is to present the same information for manipulation by com- 
puter programs. If your service exposes a set of instrument readings, the focus should 
be on providing access to the raw data, not on making human-readable graphs. Clients 
can make their own graphs, or pipe the raw data into a graph-generation service. You 
can provide graphs as a convenience, but a graph should not be the main representation 
of a set of numbers. 

Representations should be useful: that is, they should expose interesting data instead 
of irrelevant data that no one will use. A single representation should contain all relevant 
information necessary to fulfill a need. A client should not have to get several repre- 
sentations of the same resource to perform a single operation. 

That said, it's difficult to anticipate what part of your data set clients will use. When 
in doubt, expose all the state you have for a resource. This is what a Rails service does 
by default: it exposes representations that completely describe the corresponding da- 
tabase rows. 

A resource's representations should change along with its state. 

Incoming Representations 

I don't have a lot to say about incoming representations, apart from talking about 
specific formats, which I'll do in the next chapter. I will mention the two main kinds 
of incoming representations. Simple representations are usually key-value pairs: set this 
item of resource state to that value: username=leonardr. There are lots of representations 
for key-value pairs, form-encoding being the most popular. 

If your resource state is too complex to represent with key-value pairs, your service 
should accept incoming representations in the same format it uses to serve outgoing 



234 | Chapter 8: REST and ROA Best Practices 



representations. A client should be able to fetch a representation, modify it, and PUT 
it back where it found it. It doesn't make sense to have your clients understand one 
complex data format for outgoing representations and another, equally complex format 
for incoming representations. 

Service Versioning 

Web sites can (and do) undergo drastic redesigns without causing major problems, 
because their audience is made of human beings. Humans can look at a web page and 
understand what it means, so they're good at adapting to changes. Although URIs on 
the Web are not supposed to change, in practice they can (and do) change all the time. 
The consequences are serious — external links and bookmarks still point to the old URIs 
— but your everyday use of a web site isn't affected. Even so, after a major redesign, 
some web sites keep the old version around for a while. The web site's users need time 
to adapt to the new system. 

Computer programs are terrible at adapting to changes. A human being (a programmer) 
must do the adapting for them. This is why connectedness is important, and why ex- 
tensible representation formats (like Atom and XHTML) are so useful. When the cli- 
ent's options are described by hypermedia, a programmer can focus on the high-level 
semantic meaning of a service, rather than the implementation details. The implemen- 
tations of resources, the URIs to the resources, and even the hypermedia representa- 
tions themselves can change, but as long as the semantic cues are still there, old clients 
will still work. 

The mapping service from Chapter 5 was completely connected and served represen- 
tations in an extensible format. The URI to a resource followed a certain pattern, but 
you didn't need that fact to use the service: the representations were full of links, and 
the links were annotated with semantic content like "zoom_in" and "coordinates." In 
Chapter 6 I added new resources and added new features to the representations, but a 
client written against the Chapter 5 version would still work. (Except for the protocol 
change: the Chapter 5 service was served through HTTP, and the Chapter 6 service 
through HTTPS.) All the semantic cues stayed the same, so the representations still 
"meant" the same thing. 

By contrast, the bookmarking service from Chapter 7 isn't well connected. You can't 
get a representation of a user except by applying a URI construction rule I described in 
English prose. If I change that rule, any clients you wrote will break. In a situation like 
this, the service should allow for a transitional period where the old resources work 
alongside the new ones. The simplest way is to incorporate version information into 
the resources' URIs. That's what I did in Chapter 7: my URIs looked like /vl/users/ 
leonardr instead of /users/leonardr. 



Service Versioning | 235 



Even a well-connected service might need to be versioned. Sometimes a rewrite of the 
service changes the meaning of the representations, and all the clients break, even ones 
that understood the earlier semantic cues. When in doubt, version your service. 

You can use any of the methods developed over the years for numbering software re- 
leases . Your URI might designate the version asvl,orl.4.0,or 2007 - 05 - 22 . The simplest 
way to incorporate the version is to make it the first path variable: /vl/resource ver- 
sus /v2/resource. If you want to get a little fancy, you can incorporate the version 
number into the hostname: vl . service . example . com versus v2 . service . example . com. 

Ideally, you would keep the old versions of your services around until no more clients 
use them, but this is only possible in private settings where you control all the clients. 
More realistically, you should keep old versions around until architectural changes 
make it impossible to expose the old resources, or until the maintenance cost of the 
old versions exceeds the cost of actively helping your user base migrate. 

Permanent URIs Versus Readable URIs 

I think there should be an intuitive correspondence between a URI and the resource it 
identifies. REST doesn't forbid this, but it doesn't require it either. REST says that 
resources should have names, not that the names should mean anything. The 
URI /contour/Mars doesn't have to be the URI to the contour map of Mars: it could 
just as easily be the URI to the radar map of Venus, or the list of open bugs in a bug 
tracker. But making a correspondence between URI and resource is one of the most 
useful things you can do for your clients. Usability expert Jakob Nielsen recommends 
this in his essay "URL as UI" (http://www.useit.com/alertbox/990321.html) . If your URIs 
are intuitive enough, they form part of your service's user interface. A client can get 
right to the resource they want by constructing an appropriate URI, or surf your re- 
sources by varying the URIs. 

There's a problem, though. A meaningful URI talks about the resource, which means 
it contains elements of resource state. What happens when the resource state changes? 
Nobody will ever successfully rename the planet Mars (believe me, I've tried) , but towns 
change names occasionally, and businesses change names all the time. I ran into trouble 
in Chapter 6 because I used latitude and longitude to designate a "place" that turned 
out to be a moving ship. Usernames change. People get married and change their names. 
Almost any piece of resource state that might add meaning to a URI can change, break- 
ing the URI. 

This is why Rails applications expose URIs that incorporate database table IDs, URIs 
like /weblogs/4. I dissed those URIs in Chapter 7, but their advantage is that they're 
based on a bit of resource state that never changes. It's state that's totally useless to the 
client, but it never changes, and that's worth something too. 

Jakob Nielsen makes the case for meaningful URIs, but Tim Berners-Lee makes the 
case for URI opacity: "meaningless" URIs that never change. Berners-Lee's "Axioms of 



236 | Chapter 8: REST and ROA Best Practices 



Web Architecture" (http://www.w3.org/DesignIssues/Axioms.html) describes URI 
opacity like this: "When you are not dereferencing you should not look at the contents 
of the URI string to gain other information. " That is: you can use a URI as the name of 
a resource, but you shouldn't pick the URI apart to see what it says, and you shouldn't 
assume that you can vary the resource by varying the URI. Even if a URI really looks 
meaningful, you can't make any assumptions. 

This is a good rule for a general web client, because there are no guarantees about URIs 
on the Web as a whole. Just because a URI ends in ".html" doesn't mean there's an 
HTML document on the other side. But today's average RESTful web service is built 
around rules for URI construction. With URI Templates, a web service can make 
promises about whole classes of URIs that fit a certain pattern. The best argument for 
URI opacity on the programmable web is the fact that a non-opaque URI incorporates 
resource state that might change. To use another of Tim Berners-Lee's coinages, opaque 
URIs are "cool, "t 

So which is it? URI as UI, or URI opacity? For once in this book I'm going to give you 
the cop-out answer: it depends. It depends on which is worse for your clients: a URI 
that has no visible relationship to the resource it names, or a URI that breaks when its 
resource state changes. I almost always come down on the side of URI as UI, but that's 
just my opinion. 

To show you how subjective this is, I'd like to break the illusion of the authorial "I" for 
just a moment. The authors of this book both prefer informative URIs to opaque ones, 
but Leonard tries to choose URIs using the bits of resource state that are least likely to 
change. If he designed a weblog service, he'd put the date of a weblog entry in that 
entry's URI, but he wouldn't put the entry title in there. He thinks the title's too easy 
to change. Sam would rather put the title in the URI, to help with search engine opti- 
mization and to give the reader a clue what content is behind the URI. Sam would 
handle retitled entries by setting up a permanent redirect at the old URI. 

Standard Features of HTTP 

HTTP has several features designed to solve specific engineering problems. Many of 
these features are not widely known, either because the problems they solve don't come 
up very often on the human web, or because today's web browsers implement them 
transparently. When working on the programmable web, you should know about these 
features, so you don't reinvent them or prematurely give up on HTTP as an application 
protocol. 



t Hypertext Style: Cool URIs Don't Change (http://www.w3.org/Provider/Style/UK0 



Standard Features of HTTP | 237 



Authentication and Authorization 

By now you probably know that HTTP authentication and authorization are handled 
with HTTP headers — "stickers" on the HTTP "envelope." You might not know that 
these headers were designed to be extensible. HTTP defines two authentication 
schemes, but there's a standard way of integrating other authentication schemes into 
HTTP, by customizing values for the headers Authorization and WWW- Authenticate. You 
can even define custom authentication schemes and integrate them into HTTP: I'll 
show you how that's done by adapting a small portion of the WS-Security standard to 
work with HTTP authentication. But first, I'll cover the two predefined schemes. 

Basic authentication 

Basic authentication is a simple challenge/response. If you try to access a resource that's 
protected by basic authentication, and you don't provide the proper credentials, you 
receive a challenge and you have to make the request again. It's used by the del.icio.us 
web service I showed you in Chapter 2, as well as my mapping service in Chapter 6 and 
my del.icio.us clone in Chapter 7. 

Here's an example. I make a request for a protected resource, not realizing it's 
protected: 

GET /resource. html HTTP/l.l 
Host: www.example.com 

I didn't include the right credentials. In fact, I didn't include any credentials at all. The 
server sends me the following response: 

401 Unauthorized 

WWW-Authenticate: Basic realm="My Private Data" 

This is a challenge. The server dares me to repeat my request with the correct creden- 
tials. The WWW-Authenticate header gives two clues about what credentials I should 
send. It identifies what kind of authentication it's using (in this case, Basic), and it 
names a realm. The realm can be any name you like, and it's generally used to identify 
a collection of resources on a site. In Chapter 7 the realm was "Social bookmarking 
service" (I defined it in Example 7-11). A single web site might have many sets of pro- 
tected resources guarded in different ways: the realm lets the client know which au- 
thentication credentials it should provide. The realm is the what, and the authentication 
type is the how. 

To meet a Basic authentication challenge, the client needs a username and a password. 
This information might be filed in a cache under the name of the realm, or the client 
may have to prompt an end user for this information. Once the client has this infor- 
mation, username and password are combined into a single string and encoded with 
base 64 encoding. Most languages have a standard library for doing this kind of en- 
coding: Example 8-1 uses Ruby to encode a username and password. 



238 | Chapter 8: REST and ROA Best Practices 



Example 8-1. Base 64 encoding in Ruby 

#!/usr/bin/ruby 

# calculate-base64.rb 
USER="Alibaba" 
PASSWORD="open sesame" 

require 'base64' 

puts Base64.encode64("#{USER}:#{PASSW0RD}") 

# QWxpYmFiYTpvcGVuIHNlc2FtZQ== 

This seemingly random string of characters is the value of the Authorization header. 
Now I can send my request again, using the username and password as Basic auth 
credentials. 

GET /resource. html HTTP/l.l 
Host: www.example.com 

Authorization: Basic QWxpYmFiYTpvcGVuIHNlc2FtZQ== 

The server decodes this string and matches it against its user and password list. If they 
match, the response is processed further. If not, the request fails, and once again the 
status code is 401 ("Unauthorized"). 

Of course, if the server can decode this string, so can anyone who snoops on your 
network traffic. Basic authentication effectively transmits usernames and passwords in 
plain text. One solution to this is to use HTTPS, also known as Transport Level Security 
or Secure Sockets Layer. HTTPS encrypts all communications between client and serv- 
er, incidentally including the Authorization header. When I added authentication to 
my map service in Chapter 6, 1 switched from plain HTTP to encrypted HTTPS. 

Digest authentication 

HTTP Digest authentication is another way to hide the authorization credentials from 
network snoops. It's more complex than Basic authentication, but it's secure even over 
unencrypted HTTP. Digest follows the same basic pattern as Basic: the client issues a 
request, and gets a challenge. Here's a sample challenge: 

401 Unauthorized 

WWW-Authenticate: Digest realm="My Private Data", 
qop="auth", 

nonce="Occl75b9cOflb6a83lc399e26977266l", 
0paque="92eb5ffee6ae2fec3ad7lc77753l578f" 

This time, the WWW-Authenticate header says that the authentication type is Digest. The 
header specifies a realm as before, but it also contains three other pieces of information, 
including a nonce: a random string that changes on every request. 

The client's responsibility is to turn this information into an encrypted string that 
proves the client knows the password, but that doesn't actually contain the password. 
First the client generates a client-side nonce and a sequence number. Then the client 
makes a single "digest" string out of a huge amount of information: the HTTP method 



Standard Features of HTTP | 239 



and path from the request, the four pieces of information from the challenge, the user- 
name and password, the client-side nonce, and the sequence number. The formula for 
doing this is considerably more complicated than for Basic authentication (see Exam- 
ple 8-2). 

Example 8-2. HTTP digest calculation in Ruby 

#!/usr/bin/ruby 

# calculate-http-digest.rb 
require 'md5' 

#Information from the original request 

METHOD="GET" 

PATH= " /resource . html " 

# Information from the challenge 
REALM="My Private Data" 
NONCE="Occl75b9cOflb6a831c399e26977266l", 
0PAQUE="92eb5ffee6ae2fec3ad7lc77753l578f" 
Q0P="auth" 

# Information calculated by or known to the client 
NC=" 00000001" 

CN0NCE="4a8a08f09d37b73795649038408b5f33" 

USER="Alibaba" 

PASSWORD="open sesame" 

# Calculate the final digest in three steps. 

hal = MD5::hexdigest("#{USER}:#{REALM}:#{PASSW0RD}") 
ha2 = MD5 : : hexdigest ( "#{METH0D} : #{PATH} " ) 

ha3 = MD5::hexdigest("#{hal}:#{N0NCE}:#{NC}:#{CN0NCE}:#{Q0P}:#{ha2}") 
puts ha3 

# 2370039ff8a9fb83b4293210b5fb53e3 

The digest string is similar to the S3 request signature in Chapter 3. It proves certain 
things about the client. You could never produce this string unless you knew the client's 
username and password, knew what request the client was trying to make, and knew 
which challenge the server had sent in response to the first request. 

Once the digest is calculated, the client resends the request and passes back all the 
constants (except, of course, the password), as well as the final result of the calculation: 

GET /resource. html HTTP/l.l 

Host: www.example.com 

Authorization: Digest username="Alibaba", 

realm="My Private Data", 

nonce="Occl75b9cOflb6a83lc399e26977266l", 

uri="/resource . html " , 

qop=auth, 

nc=00000001, 

Cnonce="4a8a08f09d37b73795649038408b5f33", 

response="2370039ff8a9fb83b4293210b5fb53e3", 

0paque="92eb5ffee6ae2fec3ad7lc77753l578f" 



240 | Chapter 8: REST and ROA Best Practices 



The cryptography is considerably more complicated, but the process is the same as for 
HTTP Basic auth: request, challenge, response. One key difference is that even the 
server can't figure out your password from the digest. When a client initially sets a 
password for a realm, the server needs to calculate the hash of user : realm: password 
(hal in the example above), and keep it on file. That gives the server the information it 
needs to calculate the final value of ha3, without storing the user's actual password. 

A second difference is that every request the client makes is actually two requests. The 
point of the first request is to get a challenge: it includes no authentication information, 
and it always fails with a status code of 401 ("Unauthorized"). But the WWW-Authenti 
cate header includes a unique nonce, which the client can use to construct an appro- 
priate Authorization header. It makes a second request, using this header, and this one 
is the one that succeeds. In Basic auth, the client can avoid the challenge by sending its 
authorization credentials along with the first request. That's not possible in Digest. 

Digest authentication has some options I haven't shown here. Specifying qop=auth- 
int instead of qop-auth means that the calculation of ha2 above must include the re- 
quest's entity-body, not just the HTTP method and the URI path. This prevents a man- 
in-the-middle from tampering with the representations that accompany PUT and POST 
requests. 

My goal here isn't to dwell on the complex mathematics — that's what libraries are for. 
I want to demonstrate the central role the WWW-Authenticate and Authorization headers 
play in this exchange. The WWW-Authenticate header says, "Here's everything you need 
to know to authenticate, assuming you know the secret." The Authorization header 
says, "I know the secret, and here's the proof." Everything else is parameter parsing 
and a few lines of code. 

WSSEusername token 

What if neither HTTP Basic or HTTP Digest work for you? You can define your own 
standards for what goes into WWW-Authenticate and Authorization. Here's one real-life 
example. It turns out that, for a variety of technical reasons, users with low-cost hosting 
accounts can't take advantage of either HTTP Basic or HTTP Digest. $ At one time, this 
was important to a segment of the Atom community. Coming up with an entirely new 
cryptographically secure option was beyond the ability of the Atom working group. 
Instead, they looked to the WS-Security specification, which defines several different 
ways of authenticating SOAP messages with SOAP headers. (SOAP headers are the 
"stickers" on the SOAP envelope I mentioned back in Chapter 1.) They took a single 
idea — WS-Security UsernameToken — from this standard and ported it from SOAP 
headers to HTTP headers. They defined an extension to HTTP that used 
WWW-Authenticate and Authorization in a way that people with low-cost hosting ac- 
counts could use. We call the resulting extension WSSE UsernameToken, or WSSE for 

i Documented by Mark Pilgrim in "Atom Authentication" {http://wwui.xml.com/publa/2003/12/17/ 
dive.html) on xml.com. 



Standard Features of HTTP | 241 



short. (WSSE just means WS-Security Extension. Other extensions would have a claim 
to the same name, but there aren't any others right now.) 

WSSE is like Digest in that the client runs their password through a hash algorithm 
before sending it across the network. The basic pattern is the same: the client makes a 
request, gets a challenge, and formulates a response. A WSSE challenge might look like 
this: 

HTTP/l.l 401 Unauthorized 

WWW-Authenticate: WSSE realm="My Private Data", profile="UsernameToken" 

Instead of Basic or Digest, the authentication type is WSSE. The realm serves the same 
purpose as before, and the "profile" tells the client that the server expects it to generate 
a response using the UsernameToken rules (as opposed to some other rule from WS- 
Security that hasn't yet been ported to HTTP headers). The UsernameToken rules mean 
that the client generates a nonce, then hashes their password along with the nonce and 
the current date (see Example 8-3) . 

Example 8-3. Calculating a WSSE digest 

#!/usr/bin/ruby 

# calculate-wsse-digest.rb 
require 'base64' 

require 'shal' 

PASSWORD = "open sesame" 
NONCE = "EFD89F06CCB28C89", 
CREATED = "2007-04-13T09:00:OOZ" 

puts Base64.encode64(SHAl.digest("#{N0NCE}#{CREATED}#{ PASSWORD}")) 

# Z2Y59TewHV6r9BWjtHLkKfUjm2k= 

Now the client can send a response to the WSSE challenge: 

GET /resource. html HTTP/l.l 
Host: www.example.com 

Authorization: WSSE profile="UsernameToken" 
X-WSSE: UsernameToken Username="Alibaba", 

PasswordDigest="Z2Y59TewHV6r9BWjtHLkKfUjm2k=", 

Nonce="EFD89F06CCB28C89", 

Created="2007-04-13T09:00:OOZ" 

Same headers. Different authentication method. Same message flow. Different hash 
algorithm. That's all it takes to extend HTTP authentication. If you're curious, here's 
what those authentication credentials would look like as a SOAP header under the 
original WS-Security UsernameToken standard. 

<wsse : UsernameToken 

xmlns : wsse= " http : //schemas . xmlsoap . org/ws/2002/xx/ secext " 
xmlns:wsu=" http://schemas.xmlsoap.org/ws/2002/xx/utility"> 
<wsse:Username>Alibaba</wsse:Username> 
<wsse: Password Type="wsse: PasswordDigest"> 



242 I Chapter 8: REST and ROA Best Practices 



Z2Y59TewHV6r9BWjtHLkKfUjm2k= 
</wsse:Password> 

<wsse:Nonce>EFD89F06CCB28C89</wsse:Nonce> 
<wsu : Created>2007-04-13T09 : 00 : 00Z</wsu : Created> 
</wsse:UsernameToken> 

WSSE UsernameToken authentication has two big advantages. It doesn't send the 
password in the clear over the network, the way HTTP Basic does, and it doesn't require 
any special setup on the server side, the way HTTP Digest usually does. It's got one big 
disadvantage. Under HTTP Basic and Digest, the server can keep a one-way hash of 
the password instead of the password itself. If the server gets cracked, the passwords 
are still (somewhat) safe. With WSSE UsernameToken, the server must store the pass- 
word in plain text, or it can't verify the responses to its challenges. If someone cracks 
the server, they've got all the passwords. The extra complexity of HTTP Digest is meant 
to stop this from happening. Security always involves tradeoffs like these. 

Compression 

Textual representations like XML documents can be compressed to a fraction of their 
original size. An HTTP client library can request a compressed version of a represen- 
tation and then transparently decompress it for its user. Here's how it works: along 
with an HTTP request the client sends an Accept- Encoding header that says what kind 
of compression algorithms the client understands . The two standard values for Accept - 
Encoding are compress and gzip. 

GET /resource. html HTTP/l.l 
Host: www.example.com 
Accept-Encoding: gzip,compresss 

If the server understands one of the compression algorithms from Accept-Encoding, it 
can use that algorithm to compress the representation before serving it. The server sends 
the same Content-Type it would send if the representation wasn't compressed. But it 
also sends the Content- Encoding header, so the client knows the document has been 
compressed: 

200 OK 

Content-Type: text/html 
Content-Encoding: gzip 

[Binary representation goes here] 

The client decompresses the data using the algorithm given in Content- Encoding, and 
then treats it as the media type given as Content -Type. In this case the client would use 
the gzip algorithm to decompress the binary data back into an HTML document. This 
technique can save a lot of bandwidth, with very little cost in additional complexity. 

You probably remember that I think different representations of a resource should have 
distinct URIs. Why do I recommend using HTTP headers to distinguish between com- 
pressed and uncompressed versions of a representation? Because I don't think the 



Standard Features of HTTP | 243 



compressed and uncompressed versions are different representations. Compression, 
like encryption, is something that happens to a representation in transit, and must be 
undone before the client can use the representation. In an ideal world, HTTP clients 
and servers would compress and decompress representations automatically, and pro- 
grammers should not have to even think about it. Today, most web browsers auto- 
matically request compressed representations, but few programmable clients do. 

Conditional GET 

Conditional HTTP GET allows a server and client to work together to save bandwidth. 
I covered it briefly in Chapter 5, in the context of the mapping service. There, the 
problem was sending the same map tiles over and over again to clients who had already 
received them. This is a more general treatment of the same question: how can a service 
keep from sending representations to clients that already have them? 

Neither client nor server can solve this problem alone. If the client retrieves a repre- 
sentation and never talks to the server again, it will never know when the representation 
has changed. The server keeps no application state, so it doesn't know when a client 
last retrieved a certain representation. HTTP isn't a reliable protocol anyway, and the 
client might not have received the representation the first time. So when the client 
requests a representation, the server has no idea whether the client has done this before 
— unless the client provides that information as part of the application state. 

Conditional HTTP GET requires client and server to work together. When the server 
sends a representation, it sets some HTTP response headers: Last-Modified and/or 
ETag. When the client requests the same representation, it should send the values for 
those headers as If-Modified-Since and/or If-None-Match. This lets the server make a 
decision about whether or not to resend the representation. Example 8-4 gives a dem- 
onstration of conditional HTTP GET. 

Example 8-4. Make a regular GET request, then a conditional GET request 
#!/usr/bin/ruby 

# fetch-oreilly-conditional.rb 

require 'rubygems' 

require 'rest-open-uri' 

uri = 'http://www.oreilly.com' 

# Make an HTTP request and then describe the response, 
def request(uri, *args) 

begin 

response = open(uri, *args) 
rescue OpenURI: :HTTPError => e 

response = e.io 
end 

puts " Status code: #{response. status. inspect}" 
puts " Representation size: #{response.size}" 
lastjnodified = response. meta[ ' last-modified ' ] 



244 | Chapter 8: REST and ROA Best Practices 



etag = response. meta[ 'etag' ] 
puts " Last-Modified: #{last_modified}" 
puts " Etag: #{etag}" 
return lastjnodified, etag 
end 

puts "First request:" 
lastjnodified, etag = request(uri) 

puts "Second request:" 

request(uri, ' If-Modified-Since' => lastjnodified, ' If-None-Match' => etag) 

If you run that code once, it'll fetch http://www.oreilly.com twice: once normally and 
once conditionally. It prints information about each request. The printed output for 
the first request will look something like this: 

First request: 
Status code: ["200", "OK"] 
Representation size: 41123 
Last-Modified: Sun, 21 Dan 2007 09:35:19 GMT 
Etag: "7359b7-a37c-45b333d7" 

The Last-Modified and Etag headers are the ones that make HTTP conditional GET 
possible. To use them, I make the HTTP request again, but this time I use the value of 
Last-Modified as If-Modif ied-Since, and the value of ETag as If-None-Match. Here's the 
result: 

Second request: 

Status code: ["304", "Not Modified"] 
Representation size: 
Last-Modified: 

Etag: "7359b7-aOa3-45b5d90e" 

Instead of a 40-KB representation, the second request gets a 0-byte representation. 
Instead of 200 ("OK"), the status code is 304 ("Not Modified"). The second request 
saved 40 KB of bandwidth because it made the HTTP request conditional on the rep- 
resentation of http://www.oreilly.com/ actually having changed since last time. The 
representation didn't change, so it wasn't resent. 

Last-Modified is a pretty easy header to understand: it's the last time the representation 
of this resource changed. You may be able to view this information in your web browser 
by going to "view page info" or something similar. Sometimes humans check a web 
page's Last-Modified time to see how recent the data is, but its main use is in conditional 
HTTP requests. 

If-Modified-Since makes an HTTP request conditional. If the condition is met, the 
server carries out the request as it would normally. Otherwise, the condition fails and 
the server does something unusual. For If-Modified-Since, the condition is: "the rep- 
resentation I'm requesting must have changed after this date." The condition succeeds 
when the server has a newer representation than the client does. If the client and server 
have the same representation, the condition fails and the server does something un- 



Standard Features of HTTP | 245 



usual: it omits the representation and sends a status code of 304 ("Not Modified"). 
That's the server's way of telling the client: "reuse the representation you saved from 
last time." 

Both client and server benefit here. The server doesn't have to send a representation of 
the resource, and the client doesn't have to wait for it. Both sides save bandwidth. This 
is one of the tricks underlying your web browser's cache, and there's no reason not to 
use it in custom web clients. 

How does the server calculate when a representation was last modified? A web server 
like Apache has it easy: it mostly serves static files from disk, and filesystems already 
track the modification date for every file. Apache just gets that information from the 
filesystem. In more complicated scenarios, you'll need to break the representation 
down into its component parts and see when each bit of resource state was last modi- 
fied. In Chapter 7, the Last -Modified value for a list of bookmarks was the most recent 
timestamp in the list. If you're not tracking this information, the bandwidth savings 
you get by supporting Last-Modified might make it worth your while to start tracking 
it. 

Even when a server provides Last-Modified, it's not totally reliable. Let's say a client 
GETs a representation at 12:30:00.3 and sees a Last-Modified with the time "12:30:00." 
A tenth of a second later, the representation changes, but the Last-Modified time is still 
"12:30:00." If the client tries a conditional GET request using If-Modified-Since, the 
server will send a 304 ("Not Modified") response, even though the resource was modi- 
fied after the original GET. One second is not a high enough resolution to keep track 
of when a resource changes. In fact, no resolution is high enough to keep track of when 
a resource changes with total accuracy. 

This is not quite satisfactory. The world cries out for a completely reliable way of 
checking whether or not a representation has been modified since last you retrieved it. 
Enter the Etag response header. The Etag (it stands for "entity tag") is a nonsensical 
string that must change whenever the corresponding representation changes. 

The If-None-Match request header is to Etag as the If-Modif ied-Since request header 
is to Last -Modified . It's a way of making an HTTP request conditional. In this case, the 
condition is "the representation has changed, as embodied in the entity tag." It's sup- 
posed to be a totally reliable way of identifying changes between representations. 

It's easy to generate a good ETag for any representation. Transformations like the MD5 
hash can turn any string of bytes into a short string that's unique except in pathological 
cases. The problem is, by the time you can run one of those transformations, you've 
already created the representation as a string of bytes. You may save bandwidth by not 
sending the representation over the wire, but you've already done everything necessary 
to build it. 

The Apache server uses filesystem information like file size and modification time to 
generate Etag headers for static files without reading their contents. You might be able 



246 | Chapter 8: REST and R0A Best Practices 



to do the same thing for your representations: pick the data that tends to change, or 
summary data that changes along with the representation. Instead of doing an MD5 
sum of the entire representation, just do a sum of the important data. The Etag header 
doesn't need to incorporate every bit of data in the representation: it just has to change 
whenever the representation changes. 

If a server provides both Last-Modified and Etag, the client can provide both 
If-Modified-Since and If-None-Match in subsequent requests (as I did in Exam- 
ple 8-4). The server should make both checks: it should only send a new representation 
if the representation has changed and the Etag is different. 

Caching 

Conditional HTTP GET gives the client a way to refresh a representation by making a 
GET request that uses very little bandwidth if the representation has not changed. 
Caching gives the client some rough guidelines that can make it unnecessary to make 
that second GET request at all. 

HTTP caching is a complex topic, even though I'm limiting my discussion to client- 
side caches and ignoring proxy caches that sit between the client and the server. §The 
basics are these: when a client makes an HTTP GET or HEAD request, it might be able 
to cache the HTTP response document, headers and all. The next time the client is 
asked to make the same GET or HEAD request, it may be able to return the cached 
document instead of actually making the request again. From the perspective of the 
user (a human using a web browser, or a computer program using an HTTP library), 
caching is transparent. The user triggers a request, but instead of making an actual 
HTTP request, the client retrieves a cached response from the server and presents it as 
though it were freshly retrieved. I'm going to focus on three topics from the point of 
view of the service provider: how you can tell the client to cache, how you can tell the 
client not to cache, and when the client might be caching without you knowing it. 

Please cache 

When the server responds to a GET or HEAD request, it may send a date in the response 
header Expires. For instance: 

Expires: Tue, 30 Dan 2007 17:02:06 GMT 

This header tells the client (and any proxies between the server and client) how long 
the response may be cached. The date may range from a date in the past (meaning the 
response has expired by the time it gets to the client) to a date a year in the future (which 
means, roughly, "the response will never expire"). After the time specified in Expires, 
the response becomes stale. This doesn't mean that it must be removed from the cache 

§ For more detailed coverage, see section 13 of RFC 2616, and Chapter 7 of HTTP: The Definitive Guide, by 
Brian Totty and David Gourley (O'Reilly). 



Standard Features of HTTP | 247 



immediately. The client might be able to make a conditional GET request, find out that 
the response is actually still fresh, and update the cache with a new expiration date. 

The value of Expires is a rough guide, not an exact date. Most services can't predict to 
the second when a response is going to change. If Expires is an hour in the future, that 
means the server is pretty sure the response won't change for at least an hour. But 
something could legitimately happen to the resource the second after that response is 
sent, invalidating the cached response immediately. When in doubt, the client can make 
another HTTP request, hopefully a conditional one. 

The server should not send an Expires that gives a date more than a year in the future. 
Even if the server is totally confident that a particular response will never change, a year 
is a long time. Software upgrades and other events in the real world tend to invalidate 
cached responses sooner than you'd expect. 

If you don't want to calculate a date at which a response should become stale, you can 
use Cache-Control to say that a response should be cached for a certain number of 
seconds. This response can be cached for an hour: 

Cache-Control: max-age=3600 

Thank you for not caching 

That covers the case when the server would like the client to cache. What about the 
opposite? Some responses to GET requests are dynamically generated and different 
every time: caching them would be useless. Some contain sensitive information that 
shouldn't be stored where someone else might see it: caching them would cause security 
problems. Use the Cache-Control header to convey that the client should not cache the 
representation at all: 

Cache-Control: no-cache 

Where Expires is a fairly simple response header, Cache-Control header is very complex. 
It's the primary interface for controlling client-side caches, and proxy caches between 
the client and server. It can be sent as a request or as a response header, but I'm just 
going to talk about its use as a response header, since my focus is on how the server 
can work with a client-side cache. 

I already showed how specifying "max-age" in Cache-Control controls how long a re- 
sponse can stay fresh in a cache. A value of "no-cache" prevents the client from caching 
a response at all. A third value you might find useful is "private," which means that the 
response may be cached by a client cache, but not by any proxy cache between the 
client and server. 

Default caching rules 

In the absence of Expires or Cache-Control, section 13 of the HTTP standard defines a 
complex set of rules about when a client can cache a response. Unless you're going to 
set caching headers on every response, you'll need to know when a client is likely to 



248 | Chapter 8: REST and ROA Best Practices 



cache what you send, so that you can override the defaults when appropriate. I'll sum- 
marize the basic common-sense rules here. 

In general, the client may cache the responses to its successful HTTP GET and HEAD 
requests. "Success" is defined in terms of the HTTP status code: the most common 
success codes are 200 ("OK"), 301 ("Moved Permanently"), and 410 ("Gone"). 

Many (poorly-designed) web applications expose URIs that trigger side effects when 
you GET them. These dangerous URIs usually contain query strings. The HTTP stand- 
ard recommends that if a URI contains a query string, the response from that URI 
should not be automatically cached: it should only be cached if the server explicitly 
says caching is OK. If the client GETs this kind of URI twice, it should trigger the side 
effects twice, not trigger them once and then get a cached copy of the response from 
last time. 

If the client then finds itself making a PUT, POST, or DELETE request to a URI, any 
cached responses from that URI immediately become stale. The same is true of any URI 
mentioned in the Location or Content- Location of a response to a PUT, POST, or DE- 
LETE request. There's a wrinkle here, though: site A can't affect how the client caches 
responses from site B. If you POST to http://www.example.com/resource, then any 
cached response from resource is automatically stale. If the response comes back with 
a Location of http://www.example.com/resource2, then any cached response from 
http://www.example.com/resource2 is also stale. But if the Location is http://www.oreH 
ly.com/resource2, it's not OK to consider a cached response from http:// 
www.oreilly.com/resource2 to be stale. The site at www.example.com doesn't tell 
www.oreilly.com what to do. 

If none of these rules apply, and if the server doesn't specify how long to cache a re- 
sponse, the decision falls to the client side. Responses may be removed at any time or 
kept forever. More realistically, a client-side cache should consider a response to be 
stale after some time between an hour and a day. Remember that a stale response 
doesn't have to be removed from the cache: the client might make a conditional GET 
request to check whether the cached response can still be used. If the condition suc- 
ceeds, the cached response is still fresh and it can stay in the cache. 

Look-Before-You-Leap Requests 

Conditional GET is designed to save the server from sending enormous representations 
to a client that already has them. Another feature of HTTP, less often used, can save 
the client from fruitlessly sending enormous (or sensitive) representations to the serv- 
er. There's no official name for this kind of request, so I've came up with a silly name: 
look-before-you-leap requests. 

To make a LBYL request, a client sends a PUT or POST request normally, but omits 
the entity-body. Instead, the client sets the Expect request header to the string "100- 
continue". Example 8-5 shows a sample LBYL request. 



Standard Features of HTTP | 249 



Example 8-5. A sample look-before-you-leap request 

PUT /filestore/myfile.txt HTTP/l.l 
Host: example.com 
Content-length: 524288000 
Expect: 100-continue 

This is not a real PUT request: it's a question about a possible future PUT request. The 
client is asking the server: "would you allow me to PUT a new representation to the 
resource at /filestore/myfile.txt?" The server makes its decision based on the current 
state of that resource, and the HTTP headers provided by the client. In this case the 
server would examine Content-length and decide whether it's willing to accept a 500 
MB file. 

If the answer is yes, the server sends a status code of 100 ("Continue"). Then the client 
is expected to resend the PUT request, omitting the Expect and including the 500-MB 
representation in the entity-body. The server has agreed to accept that representation. 

If the answer is no, the server sends a status code of 417 ("Expectation Failed"). The 
answer might be no because the resource at /filestore/myfile.txt is write-protected, 
because the client didn't provide the proper authentication credentials, or because 500 
MB is just too big. Whatever the reason, the initial look-before-you-leap request has 
saved the client from sending 500 MB of data only to have that data rejected. Both client 
and server are better off. 

Of course, a client with a bad representation can lie about it in the headers just to get 
a status code of 100, but it won't do any good. The server won't accept a bad repre- 
sentation on the second request, any more than it would have on the first request. 

Partial GET 

Partial HTTP GET allows a client to fetch only a subset of a representation. It's usually 
used to resume interrupted downloads. Most web servers support partial GET for static 
content; so does Amazon's S3 service. 

Example 8-6 is a bit of code that makes two partial HTTP GET requests to the same 
URI. The first request gets bytes 10 through 20, and the second request gets everything 
from byte 40,000 to the end. 

Example 8-6. Make two partial HTTP GET requests 

#!/usr/bin/ruby 

# fetch-oreilly-partial.rb 

require 'rubygems' 

require 'rest-open-uri' 

uri = 'http://www.oreilly.com/' 

# Make a partial HTTP request and describe the response, 
def partial_request(uri, range) 

begin 



250 I Chapter 8: REST and R0A Best Practices 



response = open(uri, 'Range' => range) 
rescue OpenURI: :HTTPError => e 

response = e.io 
end 

puts " Status code: #{response. status. inspect}" 
puts " Representation size: #{response.size}" 
puts " Content Range: #{response.meta[ 'content-range' ]}" 
puts " Etag: #{response.meta[ ' etag' ]}" 
end 

puts "First request:" 
partial_request(uri, "bytes=10-20") 

puts "Second request:" 
partial_request(uri, "bytes=40000-") 

When I run that code I see this for the first request: 
First request: 

Status code: ["206", "Partial Content"] 
Representation size: 11 
Content Range: bytes 10-20/41123 
Etag: "7359b7-aua3-45b5d90e" 

Instead of 40 KB, the server has only sent me the 1 1 bytes I requested. Similarly for the 
second request: 
Second request: 

Status code: ["206", "Partial Content"] 
Representation size: 1123 
Content Range: bytes 40000-41122/41123 
Etag: "7359b7-aOa3-45b5d90e" 

Note that the Etag is the same in both cases. In fact, it's the same as it was back when 
I ran the conditional GET code back in Example 8-4. The value of Etag is always a value 
calculated for the whole document. That way I can combine conditional GET and 
partial GET. 

Partial GET might seem like a way to let the client access subresources of a given re- 
source. It's not. For one thing, a client can only address part of a representation by 
giving a byte range. That's not very useful unless your representation is a binary data 
structure. More importantly, if you've got subresources that someone might want to 
talk about separately from the containing resource, guess what: you've got more re- 
sources. A resource is anything that might be the target of a hypertext link. Give those 
subresources their own URIs. 

Faking PUT and DELETE 

Not all clients support HTTP PUT and DELETE. The action of an XHTML 4 form can 
only be GET or POST, and this has made a lot of people think that PUT and DELETE 
aren't real HTTP methods. Some firewalls block HTTP PUT and DELETE but not 



Faking PUT and DELETE | 251 



POST. If the server supports it, a client can get around these limitations by tunneling 
PUT and DELETE requests through overloaded POST. There's no reason these tech- 
niques can't work with other HTTP actions like HEAD, but PUT and DELETE are the 
most common. 

I recommend a tunneling technique pioneered by today's most RESTful web frame- 
works: include the "real" HTTP method in the query string. Ruby on Rails defines a 
hidden form field called jnethod which references the "real" HTTP method. If a client 
wants to delete the resource at / my/resource but can't make an HTTP DELETE request, 
it can make a POST request to /my/resource?_method=delete, or include 
_method=delete in the entity-body. Restlet uses the method variable for the same 
purpose. 

The second way is to include the "real" HTTP action in the X-HTTP-Method-Override 
HTTP request header. Google's GData API recognizes this header. I recommend ap- 
pending to the query string instead. A client that doesn't support PUT and DELETE is 
also likely to not support custom HTTP request headers. 

The Trouble with Cookies 

A web service that sends HTTP cookies violates the principle of statelessness. In fact, 
it usually violates statelessness twice. It moves application state onto the server even 
though it belongs on the client, and it stops clients from being in charge of their own 
application state. 

The first problem is simple to explain. Lots of web frameworks use cookies to imple- 
ment sessions. They set cookies that look like the Rails cookie I showed you back in 
Chapter 4: 

Set-Cookie : _session_id=clc934bbe6l68dcb904d21a7f 5644a2d ; path=/ 

That long hexadecimal number is stored as client state, but it's not application state. 
It's a meaningless key into a session hash: a bunch of application state stored on the 
server. The client has no access to this application state, and doesn't even know what's 
being stored. The client can only send its cookie with every request and let the server 
look up whatever application state the server thinks is appropriate. This is a pain for 
the client, and it's no picnic for the server either. The server has to keep this application 
state all the time, not just while the client is making a request. 

OK, so cookies shouldn't contain session IDs: that's just an excuse to keep application 
state on the server. What about cookies that really do contain application state? What 
if you serialize the actual session hash and send it as a cookie, instead of just sending 
a reference to a hash on the server? 

This can be RESTful, but it's usually not. The cookie standard says that the client can 
get rid of a cookie when it expires, or when the client terminates. This is a pretty big 
restriction on the client's control over application state. If you make 10 web requests 



252 | Chapter 8: REST and ROA Best Practices 



and suddenly the server sends you a cookie, you have to start sending this cookie with 
your future requests. You can't make those 10 precookie requests unless you quit and 
start over. To use a web browser analogy, your "Back" button is broken. You can't put 
the application in any of the states it was in before you got the cookie. 

Realistically, no client follows the cookie standard that slavishly. Your web browser 
lets you choose which cookies to accept, and lets you destroy cookies without restarting 
your browser. But clients aren't generally allowed to modify the server's cookies, or 
even understand what they mean. If the client sends application state without knowing 
what it means, it doesn't really know what request it's making. The client is just a 
custodian for whatever state the server thinks it should send. Cookies are almost always 
a way for the server to force the client to do what it wants, without explaining why. It's 
more RESTful for the server to guide the client to new application states using hyper- 
media links and forms. 

The only RESTful use of cookies is one where the client is in charge of the cookie value. 
The server can suggest values for a cookie using the Set-Cookie header, just like it can 
suggest links the client might want to follow, but the client chooses what cookie to 
send just as it chooses what links to follow. In some browser-based applications, cook- 
ies are created by the client and never sent to the server. The cookie is just a convenient 
container for application state, which makes its way to the server in representations 
and URIs. That's a very RESTful use of cookies. 

Why Should a User Trust the HTTP Client? 

HTTP authentication covers client-server authentication: the process by which the web 
service client proves to the server that it has some user's credentials. What HTTP 
doesn't cover is why the user should trust the web service client with its credentials. 
This isn't usually a problem on the human web, because we implicitly trust our web 
browsers (even when we shouldn't, like when there's spyware present on the system). 
If I'm using a web application on example.com, I'm comfortable supplying my exam 
ple.com username and password. 

But what if, behind the scenes, the web application on example, com is a client for eBay's 
web services? What if it asks me for my eBay authentication information so it can make 
hidden web service requests to ebay.com? Technically speaking, there's no difference 
between this application and a phishing site that pretends tobeebay.com, trying to trick 
me into giving it my eBay username and password. 

The standalone client programs presented in this book authenticate by encoding the 
end user's username and password in the Authorization header. That's how many web 
services work. It works fine on the human web, because the HTTP clients are our own 
trusted web browsers. But when the HTTP client is an untrusted program, possibly 
running on a foreign computer, handing it your username and password is naive at 



Why Should a User Trust the HTTP Client? | 253 



best. There's another way. Some web services attack phishing by preventing their clients 
from handling usernames and passwords at all. 

In this scenario, the end user uses her web browser (again, trusted implicitly) to get an 
authorization token. She gives this token to the web service client instead of giving her 
username and password, and the web service client sends this token in the 
Authorization header. The end user is basically delegating the ability to make web 
service calls as herself. If the web service client abuses that ability, its authorization 
token can be revoked without making the user change her password. 

Google, eBay, Yahoo ! , and Flickr all have user-client authorization systems of this type. 
Amazon's request signing, which I showed you in Chapter 3, fulfills the same function. 
There's no official standard, but all four systems are similar in concept, so I'll discuss 
them in general terms. When I need to show you specific URIs, I'll use Google's and 
Flickr's user-client authorization systems as examples. 

Applications with a Web Interface 

Let's start with the simplest case: a web application that needs to access a web service 
such as Google Calendar. It's the simplest case because the web application has the 
same user interface as the application that gives out authorization tokens: a web brows- 
er. When a web application needs to make a Google web service call, it serves an HTTP 
redirect that sends the end user to a URI at google, com. The URI might look something 
like this: 

https : // www . google . com/accounts/AuthSubRequest 
?scope=http%3A%2F%2Fwww. google. com%2Fcalendar%2Ffeeds%2F 
&next=htt p%3A%2 F%2 Fcalendar . example . com%2 Fmy 

That URI has two other URIs embedded in it as query variables. The scope variable, 
with a value of http://www.google.com/calendar/feeds/, is the base URI of the web service 
we're trying to get an authorization token for. The next variable, value http://calen 
dar. example. com/my, will be used when Google hands control of the end user's web 
browser back to the web application. 

When the end user's browser hits this URI, Google serves a web page that tells the end 
user that example.com wants to access her Google Calendar account on her behalf. If 
the user decides she trusts example . com, she authenticates with Google. She never gives 
her Google username or password to example.com. 

After authenticating the user, Google hands control back to the original web application 
by redirecting the end user's browser to a URI based on the value of the query variable 
next in the original request. In this example, next was 
http://calendar.example.com/my, so Google might redirect the end user to http://calen 
dar. example. com/my?token=IFM29SdTSpKL77INCn. The new query variable token 
contains a one-time authorization token. The web application can put this token in the 
Authorization header when it makes a web service call to Google Calendar: 



254 | Chapter 8: REST and ROA Best Practices 



Browser 
redirect 




Google auth page 

?next=calendar. example. com/my 



Display 



1 



ooO 

\\ J ^ Decision 




\ Google username/password 




Figure 8-3. How a web application gets authorization to use Google Calendar 
Authorization: AuthSub token="IFM29SdTSpKL77lNCn" 



Now the web application can make a web-service call as the end user, without actually 
knowing anything about the end user. The authentication information never leaves 
google.com, and the authorization token is only good for one request. 

Those are the basics. Google's user-client authorization mechanism has lots of other 
features. A web service client can use the one-time authorization token to get a "session 
token" that's good for more than one request. A client can digitally sign requests, sim- 
ilarly to how I signed Amazon S3 requests back in Chapter 3 . These features are different 
for every user-client authorization mechanism, so I won't dwell on them here. The point 
is this flow (shown graphically in Figure 8-3) : control moves from the web application's 
domain to the web service's domain. The user authenticates with the web service, and 
authorizes the foreign web application to act on her behalf. Then control moves back 
to the web application's domain. Now the web app has an authorization token that it 
can use in the Authorization header. It can make web service calls without knowing 
the user's username and password. 



Applications with No Web Interface 

For applications that expose a web interface, browser-based user-client authorization 
makes sense. The user is already in her web browser, and the application she's using 
is running on a faraway server. She doesn't trust the web application with her password, 
but she does trust her own web browser. But what if the web service client is a stand- 
alone application running on the user's computer? What if it's got a GUI or command- 
line interface, but it's not a web browser? 



Why Should a User Trust the HTTP Client? | 255 



There are two schools of thought on this. The first is that the end user should trust any 
client-side application as much as she trusts her web browser. Web applications run 
on an untrusted computer, but I control every web service client that runs on my com- 
puter. I can keep track of what the clients are doing and kill them if they get out of 
control. 

If you as a service designer subscribe to this philosophy, there's no need to hide the end 
user's username and password from desktop clients. They're all just as trustworthy as 
the web browser. Google takes this attitude. Its authentication mechanism for client- 
side applications (http://code.google.com/apis/accounts/AuthForInstalledApps.html) is 
different from the web-based one I described above. Both systems are based on tokens, 
but desktop applications get an authorization token by gathering the user's username 
and password and "logging in" as them — not by redirecting the user's browser to a 
Google login page. This token serves little purpose from a security standpoint. The 
client needs a token to make web service requests, but it can only get one if it knows 
the user's username and password — a far more valuable prize. 

If you don't like this, then you probably think the web browser is the only client an end 
user should trust with her username and password. This creates a problem for the 
programmer of a desktop client. Getting an authentication token means starting up a 
trusted client — the web browser — and getting the end user to visit a certain URI. For 
the Flickr service the URI might look like this: 

http : / /flickr . com/services/auth/?perms=write&api_sig=925el&api_key=1234&f rob=abcd 

The most important query variable here is frob. That's a predefined ID, obtained 
through an earlier web service call, and I'll use it in a moment. The first thing the end 
user sees is that her browser suddenly pops up and visits this URI, which shows a Flickr 
login screen. The end user gives her authentication credentials and authorizes the client 
with api_key=l234 to act on her behalf. In the Google example above, the web service 
client was the web application at example.com. Here, the web service client is the ap- 
plication running on the end user's own desktop. 

Without the frob, the desktop client at this point would have to cajole the end user to 
copy and paste the authorization token from the browser into the desktop client. But 
the client and the service agreed on a frob ahead of time, and the desktop client can 
use this frob to get the authorization token. The end user can close his browser at this 
point, and the desktop client makes a GET request to a URI that looks like this: 

http : //flickr . com/services/rest/ ?method=f lickr . auth .get Token 
&api_sig=lf348&api_key=1234&frob=abcd 

The eBay and Flickr web services use a mechanism like this: what Flickr calls a frob, 
eBay calls an runame. The end user can authorize a standalone client to make web 
service requests on her behalf, without ever telling it her username or password. I've 
diagrammed the whole process in Figure 8-4. 



256 | Chapter 8: REST and ROA Best Practices 



Web service 




Flickrusername/password 



Client-side 


1 ► 


app 






Client-side 


1 Authorized web service request 


flickr.com 





Figure 8-4. How a web application gets authorization to use Flickr 

Some mobile devices have network connectivity but no web browser. A web service 
that thinks the only trusted client is a web browser must make special allowances for 
such devices, or live with the fact that it's locking them out. 



What Problem Does this Solve? 

Despite appearances, I've gone into very little detail: just enough to give you a feel for 
the two ways an end user might delegate her authority to make web service calls. Even 
in the high-level view it's a complex system, and it's worth asking what problem it 
actually solves. After all, the end user still has to type her username and password into 
a web form, and nothing prevents a malicious application writer from sending the 
browser to a fake authorization page instead of the real page. Phishers redirect people 
to fake sign-in pages all the time, and a lot of people fall for it. So what does this 
additional infrastructure really buy? 

If you look at a bank or some other web site that's a common target of phishing attacks, 
you'll see a big warning somewhere that looks like this: "Never type in your 
mybank.com username and password unless you're using a web browser and visiting a 
URI that starts with https://www.mybank.com/." Common sense, right? It's not the 
most ironclad guarantee of security, but if you're careful you'll be all right. Yet most 
web services can't even provide this milquetoast cover. The standalone applications 



Why Should a User Trust the HTTP Client? | 257 



presented throughout this book take your service username and password as input. 
Can you trust them? If the web site at example.com wants to help you manage your 
del.icio.us bookmarks, you need to give it your del.icio.us username and password. Do 
you trust example.com? 

The human web has a universal client: the web browser. It's not a big leap of faith to 
trust a single client that runs on your computer. The programmable web has different 
clients for different purposes. Should the end user trust all those clients? The mecha- 
nisms I described in this section let the end user use her web browser — which she 
already trusts — as a way of bestowing lesser levels of trust on other clients. If a client 
abuses the trust, it can be blocked from making future web service requests. These 
strategies don't eliminate phishing attacks, but they make it possible for a savvy end 
user to avoid them, and they allow service providers to issue warnings and disclaimers. 
Without these mechanisms, it's technically impossible for the end user to tell the dif- 
ference between a legitimate client and a phishing site. They both take your password: 
the only difference is what they do with it. 



258 | Chapter 8: REST and ROA Best Practices 



CHAPTER 9 



The Building Blocks of Services 



Throughout this book I've said that web services are based on three fundamental tech- 
nologies: HTTP, URIs, and XML. But there are also lots of technologies that build on 
top of these. You can usually save yourself some work and broaden your audience by 
adopting these extra technologies: perhaps a domain-specific XML vocabulary, or a 
standard set of rules for exposing resources through HTTP's uniform interface. In this 
chapter I'll show you several technologies that can improve your web services. Some 
you're already familiar with and some will probably be new to you, but they're all 
interesting and powerful. 



Representation Formats 

What representation formats should your service actually send and receive? This is the 
question of how data should be represented, and it's an epic question. I have a few 
suggestions, which I present here in a rough order of precedence. My goal is to help 
you pick a format that says something about the semantics of your data, so you don't 
find yourself devising yet another one-off XML vocabulary that no one else will use. 

I assume your clients can accept whatever representation format you serve. The known 
needs of your clients take priority over anything I can say here. If you know your data 
is being fed directly into Microsoft Excel, you ought to serve representations in Excel 
format or a compatible CSV format. My advice also does not extend to document 
formats that can only be understood by humans. If you're serving audio files, I've got 
nothing to say about which audio format you should choose. To a first approximation, 
a programmed client finds all audio files equally unintelligible. 



XHTML 

Media type: application/xhtml+xml The common text/html media type is deprecated 
for XHTML. It's also the only media type that Internet Explorer handles as HTML. If 
your service might be serving XHTML data directly to web browsers, you might want 
to serve it as text/html. 



259 



My number-one representation recommendation is the format I've been using in my 
own services throughout this book, and the one you're probably most familiar with. 
HTML drives the human web, and XHTML can drive the programmable web. The 
XHTML standard (http://www.w3.org/TR/xhtmll/) relies on the HTML standard to do 
most of the heavy lifting (http://www.w3.org/TR/html401/). 

XHTML is HTML under a few restrictions that make every XHTML document also 
valid XML. If you know HTML, you know most of what there is to know about 
XHTML, but there are some syntactic differences, like how to present self-closing tags. 
The tag names and attributes are the same: XHTML is expressive in the same ways as 
HTML. Since the XHTML standard just points to the HTML standard and then adds 
some restrictions to it, I tend to refer to "HTML tags" and the like except where there 
really is a difference between XHTML and HTML. 

I don't actually recommend HTML as a representation format, because it can't be 
reliably parsed with an XML parser. There are many excellent and liberal HTML pars- 
ers, though (I mentioned a few in Chapter 2), so your clients have options if you can't 
or don't want to serve XHTML. Right now, XHTML is a better choice if you expect a 
wide variety of clients to handle your data. 

HTML can represent many common types of data: nested lists (tags like ul and li), 
key-value pairs (the dl tag and its children), and tabular data (the table tag and its 
children). It supports many different kinds of hypermedia. HTML does have its short- 
comings: its hypermedia forms are limited, and won't fully support HTTP's uniform 
interface until HTML 5 is released. 

HTML is also poor in semantic content. Its tag vocabulary is very computer-centric. It 
has special tags for representing computer code and output, but nothing for the other 
structured fruits of human endeavor, like poetry. One resource can link to another 
resource, and there are standard HTML attributes (rel and rev) for expressing the 
relationship between the linker and the linkee. But the HTML standard defines only 
15 possible relationships between resources, including "alternate, " "stylesteet, " "next, " 
"prev," and "glossary." See http://www.w3.Org/TR/html401/types.html#type-links for a 
complete list. 

Since HTML pages are representations of resources, and resources can be anything, 
these 15 relationships barely scratch the surface. HTML might be called upon to rep- 
resent the relationship between any two things. Of course, I can come up with my own 
values for rel and rev to supplement the official 15, but if everyone does that confusion 
will reign: we'll all pick different values to represent the same relationships. If I link my 
web page to my wife's web page, should I specify my relationship to her as husband, 
spouse, or sweetheart? To a human it doesn't matter much, but to a computer program 
(the real client on the programmable web) it matters a lot. Similarly, HTML can easily 
represent a list, and there's a standard HTML attribute (class) for expressing what kind 
of list it is. But HTML doesn't say what kinds of lists there are. 



260 | Chapter 9: The Building Blocks of Services 



This isn't HTML's fault, of course. HTML is supposed to be used by people who work 
in any field. But once you've chosen a field, everyone who works in that field should 
be able to agree on what kinds of lists there are, or what kinds of relationships can exist 
between resources. This is why people have started getting together and adding stand- 
ard semantics to XHTML with microformats. 

XHTML with Microformats 

Media type: application/xhtml+xml 

Microformats (http://microformats.org/) are lightweight standards that extend XHTML 
to give domain-specific semantics to HTML tags. Instead of reinventing data storage 
techniques like lists, microformats use existing HTML tags like ol, span, and abbr. The 
semantic content usually lives in custom values for the attributes of the tags, such as 
class, rel, and rev. Example 9-1 shows an example: someone's home telephone num- 
ber represented in the microformat known as hCard. 

Example 9-1. A telephone number represented in the hCard microformat 

<span class="tel"> 

<span class="type">home</span>: 

<span class="value">+1.4l5.555.l212</span> 
</span> 

Microformat adoption is growing, especially as more special-purpose devices get on 
the web. Any microformat document can be embedded in an XHTML page, because 
it is XHTML. A web service can serve an XHTML representation that contains micro- 
format documents, along with links to other resources and forms for creating new ones. 
This document can be automatically parsed for its microformat data, or rendered for 
human consumption with a standard web browser. 

As of the time of writing there were nine microformat specifications. The best-known 
is probably rel-nofollow, a standard value for the rel attribute invented by engineers 
at Google as a way of fighting comment spam on weblogs. Here's a complete list of 
official microformats: 

hCalendar 

A way of representing events on a calendar or planner. Based on the IETF iCalendar 
format. 

hCard 

A way of representing contact information for people and organizations. Based on 
the vCard standard defined in RFC 2426. 

rel-license 

A new value for the rel attribute, used when linking to the license terms for a 
XHTML document. For example: 



Representation Formats | 261 



<a href="http://creativecommons.org/licenses/by-nd/" rel="license"> 
Made available under a Creative Commons Attribution -NoDerivs license. 
</a> 

That's standard XHTML. The only thing the microformat does is define a meaning 
for the string license when it shows up in the rel attribute. 

rel-nofollow 

A new value for the rel attribute, used when linking to URIs without necessarily 
endorsing them. 

rel-tag 

A new value for the rel attribute, used to label a web page according to some 
external classification system. 

VoteLinks 

A new value for the rev attribute, an extension of the idea behind rel-nofollow. 
VoteLinks lets you say how you feel about the resource you're linking to by casting 
a "vote." For instance: 

<a rev="vote-for" href="http://www. example. com">The best webpage ever.</a> 
<a rev="vote-against" href="http://example.com/"> 
A shameless ripoff of www. example. com</a> 

XFN 

Stands for XHTML Friends Network. A new set of values for the rel attribute, for 
capturing the relationships between people. An XFN value for the rel attribute 
captures the relationship between this "person" resource and another such re- 
source. To bring back the "Alice" and "Bob" resources from "Relationships Be- 
tween Resources" in Chapter 8, an XHTML representation of Alice might include 
this link: 

<a rel="spouse" href="Bob">Bob</a> 

XMDP 

Stands for XHTML Meta Data Profiles. A way of describing your custom values 
for XHTML attributes, using the XHTML tags for definition lists: DL, DD, and DT. 
This is a kind of meta-microformat: a microformat like rel-tag could itself be 
described with an XMDP document. 

XOXO 

Stands (sort of) for Extensible Open XHTML Outlines. Uses XHTML's list tags to 
represent outlines. There's nothing in XOXO that's not already in the XHTML 
standard, but declaring a document (or a list in a document) to be XOXO signals 
that a list is an outline, not just a random list. 

Those are the official microformat standards; they should give you an idea of what 
microformats are for. As of the time of writing there were also about 10 microformat 
drafts and more than 50 discussions about possible new microformats. Here are some 
of the more interesting drafts: 



262 | Chapter 9: The Building Blocks of Services 



geo 

A way of marking up latitude and longitude on Earth. This would be useful in the 
mapping application I designed in Chapter 5. I didn't use it there because there's 
still a debate about how to represent latitude and longitude on other planetary 
bodies: extend geo or define different microformats for each body? 

hAtom 

A way of representing in XHTML the data Atom represents in XML. 

hResume 

A way of representing resumes. 

hReview 

A way of representing reviews, such as product reviews or restaurant reviews. 
xFolk 

A way of representing bookmarks. This would make an excellent representation 
format for the social bookmarking application in Chapter 7. I chose to use Atom 
instead because it was less code to show you. 

You get the idea. The power of microformats is that they're based on HTML, the most 
widely-deployed markup format in existence. Because they're HTML, they can be em- 
bedded in web pages. Because they're also XML, they can be embedded in XML docu- 
ments. They can be understood at various levels by human beings, specialized micro- 
format processors, dumb HTML processors, and even dumber XML processors. 

Even if the microformats wiki (http://microformats.org/wiki/Main_Page) shows no mi- 
croformat standard or draft for your problem space, you might find an open discussion 
on the topic that helps you clarify your data structures. You can also create your own 
microformat (see "Ad Hoc XHTML" later in this chapter). 

Atom 

Media type: application/atom+xml 

Atom is an XML vocabulary for describing lists of timestamped entries. The entries can 
be anything, but they usually contain pieces of human-authored text like you'd see on 
a weblog or a news site. Why should you use an Atom list instead of a regular XHTML 
list? Because Atom provides special tags for conveying the semantics of publishing: 
authors, contributors, languages, copyright information, titles, categories, and so on. 
(Of course, as I mentioned earlier, there's a microformat called hAtom that brings all 
of these semantics into XHTML.) Atom is a useful XML vocabulary because so many 
web services are, in the broad sense, ways of publishing information. What's more, 
there are a lot of web service clients that understand the semantics of Atom documents. 
If your web service is addressable and your resources expose Atom representations, 
you've immediately got a huge audience. 

Atom lists are called feeds, and the items in the lists are called entries. 



Representation Formats | 263 



Some feeds are written in some version of RSS, a different XML vo- 
cabulary with similar semantics. All versions of RSS have the same basic 
I structure as Atom: a feed that contains a number of entries. There are 
a number of variants of RSS but you shouldn't have to worry about it at 
all. Today, every major tool for consuming feeds understands Atom. 



These days, most weblogs and news sites expose a special resource whose representa- 
tion is an Atom feed. The entries in the feed describe and link to other resources: weblog 
entries or news stories published on the site. You, the client, can consume these re- 
sources with a feed reader or some other external program. In Chapter 7, 1 represented 
lists of bookmarks as Atom feeds. Example 9-2 shows a simple Atom feed document. 

Example 9-2. A simple Atom feed containing one entry 

<?xml version="l.O" encoding="utf-8"?> 
<feed xmlns=" http://www.w3.org/2005/Atom"> 
<title>RESTful News</title> 

<link rel="alternate" href="http://example.com/RestfulNews" /> 
<updated>2007-04-14T20:00:39Z</updated> 
<authorxname> Leonard Richardson</namex/author> 
<contributorxname>Sam Ruby</namex/contributor> 
<id>urn : Ic6627a0-8e3f-0129-bla6-003065546f l8</id> 

<entry> 

<title>New Resource Will Respond to PUT, City Says</title> 
<link rel="edit" href="http://example.com/RestfulNews/l04" /> 
<id>urn : 239b2f40-8e3f -0129-bla6-003065546f l8</id> 
<updated>2007-04-14T20:00:39Z</updated> 

<summary> 

After long negotiations, city officials say the new resource 
being built in the town square will respond to PUT. Earlier 
criticism of the proposal focused on the city's plan to modify 
the resource through overloaded POST. 
</summary> 

<category scheme=" http://www.example.com/categories/RestfulNews" 
term="local" label="Local news" /> 

</entry> 
</feed> 

In that example you can see some of the tags that convey the semantics of publishing: 
author, title, link, summary, updated, and so on. The feed as a whole is a joint project: 
it has an author tag and a contributor tag. It's also got a link tag that points to an 
alternate URI for the underlying "feed" resource: the news site. The single entry has no 
author tag, so it inherits author information from the feed. The entry does have its own 
link tag, which points to http://www.example.com/RestfulNews/104. That URI identi- 
fies the entry as a resource in its own right. The entry also has a textual summary of the 
story. To get the remainder, the client must presumably GET the entry's URI. 



264 I Chapter 9: The Building Blocks of Services 



An Atom document is basically a directory of published resources. You can use Atom 
to represent photo galleries, albums of music (maybe a link to the cover art plus one to 
each track on the album), or lists of search results. Or you can omit the LINK tags and 
use Atom as a container for original content like status reports or incoming emails. 
Remember: the two reasons to use Atom are that it represents the semantics of pub- 
lishing, and that a lot of existing clients can consume it. 

If your application almost fits in with the Atom schema, but needs an extra tag or two, 
there's no problem. You can embed XML tags from other namespaces in an Atom feed. 
You can even define a custom namespace and embed its tags in your Atom feeds. This 
is the Atom equivalent of XHTML microformats: your Atom feeds can use conventions 
not defined in Atom, without becoming invalid. Clients that don't understand your tag 
will see a normal Atom feed with some extra mysterious data in it. 

OpenSearch 

OpenSearch (http://www.opensearch.org/) is one XML vocabulary that's commonly 
embedded in Atom documents. It's designed for representing lists of search results. 
The idea is that a service returns the results of a query as an Atom feed, with the indi- 
vidual results represented as Atom entries. But some aspects of a list of search results 
can't be represented in a stock Atom feed: the total number of results, for instance. So 
OpenSearch defines three new elements, in the opensearch namespace:* 

totalResults 

The total number of results that matched the query. 
itemsPerPage 

How many items are returned in a single "page" of search results, 
startindex 

If all the search results are numbered from zero to totalResults, then the first result 
in this feed document is entry number startindex. When combined with 
itemsPerPage you can use this to figure out what "page" of results you're on. 

SVG 

Media type: image/svg+xml 

Most graphic formats are just ways of laying pixels out on the screen. The underlying 
content is opaque to a computer: it takes a skilled human to modify a graphic or reuse 
part of one in another. Scalable Vector Graphics is an XML vocabulary that makes it 
possible for programs to understand and manipulate graphics. It describes graphics in 
terms of primitives like shapes, text, colors, and effects. 



OpenSearch also defines a simple control flow: a special kind of resource called a "description document." 
I'm not covering OpenSearch description documents in this book, mainly for space reasons. 



Representation Formats | 265 



It would be a waste of time to represent a photograph in SVG, but using it to represent 
a graph, a diagram, or a set of relationships gives a lot of power to the client. SVG images 
can be scaled to arbitrary size without losing any detail. SVG diagrams can be edited 
or rearranged, and bits of them can be seamlessly snipped out and incorporated into 
other graphics. In short, SVG makes graphic documents work like other sorts of docu- 
ments. Web browsers are starting to get support for SVG: newer versions of Firefox 
support it natively. 

Form-Encoded Key-Value Pairs 

Media type: application/x-www-form-urlencoded 

I covered this simple format in Chapter 6. This format is mainly used in representations 
the client sends to the server. A filled-out HTML form is represented in this format by 
default, and it's an easy format for an Ajax application to construct. But a service can 
also use this format in the representations it sends. If you're thinking of serving comma- 
separated values or RFC 822-style key-value pairs, try form-encoded values instead. 
Form-encoding takes care of the tricky cases, and your clients are more likely to have 
a library that can decode the document. 

JSON 

Media type: application/ j son 

JavaScript Object Notation is a serialization format for general data structures. It's 
much more lightweight and readable than an equivalent XML document, so I recom- 
mend it for most cases when you're transporting a serialized data structure rather than 
a hypermedia document. 

I introduced JSON in "JSON Parsers: Handling Serialized Data" in Chapter 2, and 
showed a simple JSON document in Example 2-11. Example 9-3 shows a more complex 
JSON document: a hash of lists. 

Example 9-3. A complex data type in ] SON format 
{"a":["b'7'c"], "1":[2,3]} 

As I show in Chapter 11, JSON has special advantages when it comes to Ajax applica- 
tions. It's useful for any kind of application, though. If your data structures are more 
complex than key-value pairs, or you're thinking of defining an ad hoc XML format, 
you might find it easier to define a JSON structure of nested hashes and arrays. 

RDF and RDFa 

The Resource Description Framework (http://www.w3.org/RDF/) is a way of repre- 
senting knowledge about resources. Resource here means the same thing as in Re- 
source-Oriented-Architecture: a resource is anything important enough to have a URL 



266 | Chapter 9: The Building Blocks of Services 



In RDF, though, the URIs might not be http: URIs. Abstract URI schemas like isbn: 
(for books) and urn: (for just about anything) are common. Example 9-4 is a simple 
RDF assertion, which claims that the title of this book is RESTful Web Services. 

Example 9-4. An RDF assertion 

<span about="isbn:9780596529260" property="dc:title"> 
RESTful Web Services 
</span> 

There are three parts to an RDF assertion, or triple, as they're called. There's the sub- 
ject, a resource identifier: in this case, isbn: 9780596529260. There's the predicate, which 
identifies a property of the resource: in this case, dc: title. Finally there's the object, 
which is the value of the property: in this case, "RESTful Web Services. " The assertion 
as a whole reads: "The book with ISBN 9780596529260 has a title of 'RESTful Web 
Services.'" 

I didn't make up the isbn : URI space: it's a standard way of addressing books as re- 
sources. I didn't make up the dc: title predicate, either. That comes from the Dublin 
Core Metadata Initiative (http://www.dublincore.org/documents/dcmi-terms/). DCMI 
defines a set of useful predicates that apply to published works like books and weblogs. 
An automated client that understands the Dublin Core can scan RDF documents that 
use those terms, evaluate the assertions they contain, and even make logical deductions 
about the data. 

Example 9-4 looks a lot like an XHTML snippet, because that's what it is. There are a 
couple ways of representing RDF assertions, and I've chosen to show you RDFa (http:// 
rdfa. info/about), a microformat-like standard for embedding RDF in XHTML. RDF/ 
XML is a more popular RDF representation format, but I think it makes RDF look more 
complicated than it is, and it's difficult to integrate RDF/XML documents into the web. 
RDF/A documents can go into XHTML files, just like microformat documents. How- 
ever, since RDFa takes some ideas from the unreleased XHTML 2 standard, a document 
that includes it won't be valid XHTML for a while. A third way of representing RDF 
assertions is eRDF (http://research.talis.com/2005/erdf/wiki), which results in valid 
XHTML. 

RDF in its generic form is the basis for the W3C's Semantic Web project. On the human 
web, there are no standards for how we talk about the resources we link to. We describe 
resources in human language that's difficult or impossible for machines to understand. 
RDF is a way of constraining human speech so that we talk about resources using a 
standard vocabulary — not one that machines "understand" natively, but one they can 
be programmed to understand. A computer program doesn't understand the Dublin 
Core's "dc: title" any more than it understands "title." But if everyone agrees to use 
"dctitle," we can program standard clients to reason about the Dublin Core in con- 
sistent ways. 

Here's the thing: I think microformats do a good job of adding semantics to the web 
we already have, and they add less complexity than RDF's general subject-predicate- 



Representation Formats | 267 



object form. I recommend using RDF only when you want interoperability with existing 
RDF processors, or are treating RDF as a general-purpose microformat for representing 
assertions about resources. 

One very popular use of RDF is FOAF (http://www.foaf-project.org/) , a way of repre- 
senting information about human beings and the relationships between them. 

Framework-Specific Serialization Formats 

Media type: application/xml 

Fm talking here about informal XML vocabularies used by frameworks like Ruby's 
ActiveRecord and Python's Django to serialize database objects as XML. I gave an 
example back in Example 7-4. It's a simple data structure: a hash or a list of hashes. 

These representation formats are very convenient if you happen to be writing a service 
that gives you access to one. In Rails, you can just call to_xml on an ActiveRecord object 
or a list of such objects. The Rails serialization format is also useful if you're not using 
Rails, but you want your service to be usable by ActiveResource clients. Otherwise, I 
don't really recommend these formats, unless you're just trying to get something up 
and running quickly (as I am in Chapters 7 and 12). The major downside of these 
formats is that they look like documents, but they're really just serialized data struc- 
tures. They never contain hypermedia links or forms. 

Ad Hoc XHTML 

Media type: application/xhtml+xml 

If none of the work that's already been done fits your problem space. . . well, first, think 
again. Just as you should think again before deciding you can't fit your resources into 
HTTP's uniform interface. If you think your resources can't be represented by stock 
HTML or Atom or RDF or JSON, there's a good chance you haven't looked at the 
problem in the right way. 

But it's quite possible that your resources won't fit any of the representation formats 
I've mentioned so far. Or maybe you can represent most of your resource state with 
XHTML plus some well-chosen microformats, but there's still something missing. The 
next step is to consider creating your own microformat. 

The high-impact way of creating a microformat is to go through the microformat proc- 
ess (http://microformats.org/wiki/process), hammer it out with other microformat en- 
thusiasts, and get it published as an official microformat. This is most appropriate when 
lots of people are trying to represent the same kind of data. Ideally, you're in a situation 
where the human web is littered with ad hoc HTML representations of the data, and 
where there are already a couple of big standards that can serve as a model for a more 
agile microformat. This is how the hCard and hCalendar microformats were developed. 
There were many people trying to put contact information and upcoming events on 



268 | Chapter 9: The Building Blocks of Services 



the human web, and preexisting standards (vCard and iCalendar) to steal ideas from. 
The representation of "places on a map" that I devised in Chapter 5 might be a starting 
point for an official microformat. There are lots of mapping sites on the human web, 
and lots of heavyweight standards for representing GIS data. If I wanted to build a 
microformat, I'd have a lot of ideas to work from. 

The low-impact way of creating a microformat is to add semantic content to the 
XHTML you were going to write anyway. This is suitable for representation formats 
that no one else is likely to use, or as a starting point so you can get a real web service 
running while you're going through the microformat process. The representation of 
the list of planets from Chapter 5 works better as an ad hoc set of semantics than as an 
official microformat. All it's doing is saying that one particular list is a list of planets. 

The microformat design patterns (http://microformats.Org/wiki/Main_Page#De 
sign patterns) and naming principles (http://microformats.org/wiki/naming-principles) 
give a set of sensible general rules for adding semantics to HTML. Their advice is useful 
even if you're not trying to create an official microformat. The semantics you choose 
for your "micromicroformat" won't be standardized, but you can present them in a 
standard way: the way microformats do it. Here are some of the more useful patterns. 

• If there's an HTML tag that conveys the semantics you want, use it. To represent 
a set of key-value pairs, use the dl tag. To represent a list, use one of the list tags. 
If nothing fits, use the span or div tag. 

• Give a tag additional semantics by specifying its class attribute. This is especially 
important for span and div, which have no real meaning on their own. 

• Use the rel attribute in a link to specify another resource's relationship to this one. 
Use the rev attribute to specify this page's relationship to another one. If the rela- 
tionship is symmetric, use rel. See "Hypermedia Technologies" later in this chapter 
for more on this. 

• Consider providing an XMDP file that describes your custom values for class, 
rel, and rev. 

Other XML Standards and Ad Hoc Vocabularies 

Media type: application/xml 

In addition to XHTML, Atom, and SVG, there are a lot of specialized XML vocabularies 
I haven't covered: MathML, OpenDocument, Chemical Markup Language, and so on. 
There are also specialized vocabularies you can use in RDF assertions, like Dublin Core 
and FOAF. A web service might serve any of these vocabularies as standalone repre- 
sentations, embed them into Atom feeds, or even wrap them in SOAP envelopes. If 
none of these work for you, you can define a custom XML vocabulary to represent your 
resource state, or maybe the parts that Atom doesn't cover. 



Representation Formats | 269 



Although I've presented this as the last resort, that's certainly not the common view. 
People come up with custom XML vocabularies all the time: that's how there got to be 
so many of them. Almost every real web service mentioned in this book exposes its 
representations in a custom XML vocabulary. Amazon S3, Yahool's search APs, and 
the del.icio.us API all serve representations that use custom XML vocabularies, even 
though they could easily serve Atom or XHTML and reuse an existing vocabulary. 

Part of this is tech culture. The microformats idea is fairly new, and a custom XML 
vocabulary still looks more "official." But this is an illusion. Unless you provide a sche- 
ma definition for your vocabulary, your custom tags have exactly the same status as a 
custom value for the HTML "class" attribute. Even a definition does nothing but codify 
the vocabulary you made up: it doesn't confer any legitimacy. Legitimacy can only come 
"from the consent of the governed": from other people adopting your vocabulary. 

That said, there is a space for custom XML vocabularies. It's usually easy to use XHTML 
instead of creating your own XML tags, but it's not so easy when you need tags with a 
lot of custom attributes. In that situation, a custom XML vocabulary makes sense. All 
I ask is that you seriously think about whether you really need to define a new XML 
vocabulary for a given problem. It's possible that in the future, people will err in the 
opposite direction, and create ad hoc microformats when they shouldn't. Then I'll urge 
caution before creating a microformat. But right now, the problem is too many ad hoc 
XML vocabularies. 

Encoding Issues 

It's a global world (I actually heard someone say that once), and any service you expose 
must deal with the products of people who speak different languages from you and use 
different writing systems. You don't have to understand all of these languages, but to 
handle multilingual data without mangling it, you do need to know something about 
character encodings: the conventions that let us represent human-readable text as 
strings of bytes. 

Every text file you've ever created has some character encoding, even though you prob- 
ably never made a decision about which encoding to use (it's usually a system property) . 
In the United States the encoding is usually UTF-8, US-ASCII, or Windows-1252. In 
western Europe it might also be ISO 8859-1. The default for HTML on the web is ISO 
8859-1, which is almost but not quite the same as Windows-1252. Japanese documents 
are commonly encoded with EUC-JP, Shift_JIS, or UTF-8. If you're curious about what 
character encodings are used in different places, most web browsers list the encodings 
they understand. My web browser supports five different encodings for simplified Chi- 
nese, five for Hebrew, nine for the Cyrillic alphabet, and so on. Most of these encodings 
are mutually incompatible, even when they encode the same language. It's insane! 

Fortunately there is a way out of this confusion. We as a species have come up with 
Unicode, a way of representing every human writing system. Unicode isn't a character 
encoding, but there are two good encodings for it: UTF-8 (more efficient for alphabetic 



270 | Chapter 9: The Building Blocks of Services 



languages like English) and UTF-16 (more efficient for logographic languages like Jap- 
anese) . Either of these encodings can handle text written in any combination of human 
languages. The best single decision you can make when handling multilingual data is 
to keep all of your data in one of these encodings: probably UTF-8 unless you live or do 
a lot of business in east Asia, then maybe UTF-16 with a byte-order mark. 

This might be as simple as making a decision when you start the project, or you may 
have to convert an existing database. You might have to install an encoding converter 
to work on incoming data, or write encoding detection code. The Universal Encoding 
Detector (http://chardet.feedparser.org/) is an excellent autodetection library for Py- 
thon. It's got a Ruby port, available as the chardet gem. It might be easy or difficult. 
But once you're keeping all of this data in one of the Unicode encodings, most of your 
problems will be over. When your clients send you data in a weird encoding, you'll be 
able to convert it to your chosen UTF-* encoding. If they send data that specifies no 
format at all, you'll be able to guess its encoding and convert it, or reject it as 
unintelligible. 

The other half of the equation is communicating with your clients: how do you tell 
them which encoding you're using in your outgoing representations? Well, XML lets 
you specify a character encoding on the very first line: 
<?xml version="l.O" encoding="UTF-8"?> 

All but one of my recommended representation formats is based on XML, so that solves 
most of the problem. But there is an encoding problem with that one outlier, and there's 
a further problem in the relationship between XML and HTTP. 

XML and HTTP: Battle of the encodings 

An XML document can and should define a character encoding in its first line, so that 
the client will know how to interpret the document. An HTTP response can and should 
specify a value for the Content -Type response header, so that the client knows it's being 
given an XML document and not some other kind. But the Content-type can also specify 
a document character encoding with "charset," and this encoding might conflict with 
what it actually says in the document. 

Content-Type: application/xml; charset="ebcdic-fr-297+euro" 

<?xml version="l.O" encoding="UTF-8"?> 

Who wins? Surprisingly, HTTP's character encoding takes precedence over the encod- 
ing in the document itself. +If the document says "UTF-8" and Content-Type says "ebc- 
dic-fr-297+euro," then extended French EBCDIC it is. Almost no one expects this kind 
of surprise, and most programmers write code first and check the RFCs later. The result 
is that the character encoding, as specified in Content -Type, tends to be unreliable. Some 



t This is specified, and argued for, in RFC 3023. 



Representation Formats | 271 



servers claim everything they serve is UTF-8, even though the actual documents say 
otherwise. 

When serving XML documents, I don't recommend going out of your way to send a 
character encoding as part of Content-type. You can do it if you're absolutely sure 
you've got the right encoding, but it won't do much good. What's really important is 
that you specify a document encoding. (Technically you can do without a document 
encoding if you're using UTF-8, or if you're using UTF-16 with a byte-order mark. But 
if you have that much control over the data, you should be able to specify a document 
encoding.) If you're writing a web service client, be aware that any character encoding 
specified in Content-Type may be incorrect. Use common sense to decide which en- 
coding declaration to believe, rather than relying on a counterintuitive rule from an 
RFC a lot of people haven't read. 

Another note: when you serve XML documents, you should serve them with a media 
type of application/xml, not text/xml. If you serve a document as text/xml with no 
charset, the correct client behavior is to totally ignore the encoding specified in the 
XML document and interpret the XML document as US-ASCII. tAvoid these compli- 
cations altogether by always serving XML as application/xml, and always specifying 
an encoding in the first line of the XML documents you generate. 

The character encoding of a JSON document 

I didn't mention plain text in my list of recommended representation formats, mostly 
because plain text is not a structured format, but also because the lack of structure 
means there's no way to specify the character encoding of "plain text." JSON is a way 
of structuring plain text, but it doesn't solve the character encoding problem. Fortu- 
nately, you don't have to solve it yourself: just follow the standard convention. 

RFC 4627 states that a JSON file must contain Unicode characters, encoded in one of 
the UTF-* encodings. Practically, this means either UTF-8, or UTF-16 with a byte-order 
mark. Plain US-ASCII will also work, since ASCII text happens to be valid UTF-8. Given 
this restriction, a client can determine the character encoding of a JSON document by 
looking at the first four bytes (the details are in RFC 4627), and there's no need to 
specify an explicit encoding. You should follow this convention whenever you serve 
plain text, not just JSON. 

Prepackaged Control Flows 

Not only does HTTP have a uniform interface, it has a standard set of response codes 
— possible ways a request can turn out. Though resources can be anything at all, they 
usually fall into a few broad categories: database tables and their rows, publications 

t Again, according to RFC 3023, which few developers have read. For a lucid explanation of these problems, 
see Mark Pilgrim's article "XML on the Web Has Failed" {http://wivw.xml.eom/pub/a/2004/07/21/dive.html). 



272 | Chapter 9: The Building Blocks of Services 



and the articles they publish, and so on. When you know what sort of resource a service 
exposes, you can often anticipate the possible responses to an HTTP request without 
knowing too much about the resource. 

In one sense the standard HTTP response codes (see Appendix B) are just a suggested 
control flow: a set of instructions about what to do when you get certain kinds of 
requests. But that's pretty vague advice, and we can do better. Here I present several 
prepackaged control flows: patterns that bring together advice about resource design, 
representation formats, and response codes to help you design real-world services. 

General Rules 

These snippets of control flow can be applied to almost any service. I can make very 
general statements about them because they have nothing to do with the actual nature 
of your resources. All I'm doing here is picking out a few important HTTP status codes 
and telling you when to use them. 

You should be able to implement these rules as common code that runs before your 
normal request handling. In Example 7-111 implemented most of them as Rails filters 
that run before certain actions, or as Ruby methods that short-circuit a request unless 
a certain condition is met. 

If the client tries to do something without providing the correct authorization, send a 
response code of 40 1 ("Unauthorized") along with instructions for correctly formatting 
the Authorization header. 

If the client tries to access a URI that doesn't correspond to any existing resource, send 
a response code of 404 ("Not Found"). The only possible exception is when the client 
is trying to PUT a new resource to that URI. 

If the client tries to use a part of the uniform interface that a resource doesn't support, 
send a response code of 405 ("Method Not Allowed"). This is the proper response 
when the client tries to DELETE a read-only resource. 

Database-Backed Control Flow 

In many web services there's a strong connection between a resource and something 
in a SQL database: a row in the database, a table, or the database as a whole. These 
services are so common that entire frameworks like Rails are oriented to making them 
easy to write. Since these services are similar in design, it makes sense that their control 
flows should also be similar. 

For instance, if an incoming request contains a nonsensical representation, the proper 
response is almost certainly 415 ("Unsupported Media Type") or 400 ("Bad Request") . 
It's up to the application to decide which representations make sense, but the HTTP 
standard is pretty strict about the possible responses to "nonsensical representation." 



Prepackaged Control Flows | 273 



With this in mind, I've devised a standard control flow for the uniform interface in a 
database-backed application. It runs on top of the general rules I mentioned in the 
previous section. I used this control flow in the controller code throughout Chap- 
ter 7. Indeed, if you look at the code in that chapter you'll see that I implemented the 
same ideas multiple times. There's space in the REST ecosystem for a higher-level 
framework that implements this control flow, or some improved version of it. 

GET 

If the resource can be identified, send a representation along with a response code of 
200 ("OK"). Be sure to support conditional GET! 

PUT 

If the resource already exists, parse the representation and turn it into a series of changes 
to the state of this resource. If the changes would leave the resource in an incomplete 
or inconsistent state, send a response code of 400 ("Bad Request"). 

If the changes would cause the resource state to conflict with some other resource, send 
a response code of 409 ("Conflict"). My social bookmarking service sends a response 
code of 409 if you try to change your username to a name that's already taken. 

If there are no problems with the proposed changes, apply them to the existing resource. 
If the changes in resource state mean that the resource is now available at a different 
URI, send a response code of 301 ("Moved Permanently") and include the new URI in 
the Location header. Otherwise, send a response code of 200 ("OK"). Requests to the 
old URI should now result in a response code of 301 ("Moved Permanently"), 404 
("Not Found"), or 410 ("Gone"). 

There are two ways to handle a PUT request to a URI that doesn't correspond to any 
resource. You can return a status code of 404 ("Not Found"), or you can create a 
resource at that URI. If you want to create a new resource, parse the representation and 
use it to form the initial resource state. Send a response code of 201 ("Created"). If 
there's not enough information to create a new resource, send a response code of 400 
("Bad Request"). 

POST for creating a new resource 

Parse the representation, pick an appropriate URI, and create a new resource there. 
Send a response code of 201 ("Created") and include the URI of the new resource in 
the Location header. If there's not enough information provided to create the resource, 
send a response code of 400 ("Bad Request"). If the provided resource state would 
conflict with some existing resource, send a response code of 409 ("Conflict"), and 
include a Location header that points to the problematic resource. 



274 | Chapter 9: The Building Blocks of Services 



POST for appending to a resource 

Parse the representation. If it doesn't make sense, send a response code of 400 ("Bad 
Request") . Otherwise, modify the resource state so that it incorporates the information 
in the representation. Send a response code of 200 ("OK"). 

DELETE 

Send a response code of 200 ("OK"). 

The Atom Publishing Protocol 

Earlier I described Atom as an XML vocabulary that describes the semantics of pub- 
lishing: authors, summaries, categories, and so on. The Atom Publishing Protocol 
(http://tools.ietf.org/wg/atompub/) (APP) defines a set of resources that capture the 
process of publishing: posting a story to a site, editing it, assigning it to a category, 
deleting it, and so on. 

The obvious applications for the APP are those for Atom and online publishing in 
general: weblogs, photo albums, content management systems, and the like. The APP 
defines four kinds of resources, specifies some of their behavior under the uniform 
interface, and defines the representation documents they should accept and serve. It 
says nothing about URI design or what data should go into the documents: that's up 
to the individual application. 

The APP takes HTTP's uniform interface and puts a higher-level uniform interface on 
top of it. Many kinds of applications can conform to the APP, and a generic APP client 
should be able to access all of them. Specific applications can extend the APP by ex- 
posing additional resources, or making the APP resources expose more of HTTP's uni- 
form interface, but they should all support the minimal features mentioned in the APP 
standard. 

The ultimate end of the APP is to serve Atom documents to the end user. Of course, 
the Atom documents are just the representations of underlying resources. The APP 
defines what those resources are. It defines two resources that correspond to Atom 
documents, and two that help the client find and modify APP resources. 

Collections 

An APP collection is a resource whose representation is an Atom feed. The document 
in Example 9-2 has everything it takes to be a representation of an Atom collection. 
There's no necessary difference between an Atom feed you subscribe to in your feed 
reader, and an Atom feed that you manipulate with an APP client. A collection is just 
a list or grouping of pieces of data: what the APP calls members. The APP is heavily 
oriented toward manipulating "collection" type resources. 



Prepackaged Control Flows | 275 



The APP defines a collection's response to GET and POST requests. GET returns a 
representation: the Atom feed. POST adds a new member to the collection, which 
(usually) shows up as a new entry in the feed. Maybe you can also DELETE a collection, 
or modify its settings with a PUT request. The APP doesn't cover that part: it's up to 
your application. 

Members 

An APP collection is a collection of members. A member corresponds roughly to an 
entry in an Atom feed: a weblog entry, a news article, or a bookmark. But a member 
can also be a picture, song, movie, or Word document: a binary format that can't be 
represented in XML as part of an Atom document. 

A client creates a member inside a collection by POSTing a representation of the mem- 
ber to the collection URI. This pattern should be familiar to you by now: the member 
is created as a subordinate resource of the collection. The server assigns the new mem- 
ber a URI. The response to the POST request has a response code of 201 ("Created"), 
and a Location header that lets the client know where to find the new resource. 

Example 9-5 shows an Atom entry document: a representation of a member. This is 
the same sort of entry tag I showed you in Example 9-2, presented as a standalone XML 
document. POSTing this document to a collection creates a new member, which starts 
showing up as a child of the collection's feed tag. A document like this one might be 
how the entry tag in Example 9-2 got where it is today. 

Example 9-5. A sample Atom entry document, suitable for POSTing to a collection 

<?xml version="l.O" encoding="utf-8"?> 
<entry> 

<title>New Resource Will Respond to PUT, City Says</title> 
<summary> 

After long negotiations, city officials say the new resource 
being built in the town square will respond to PUT. Earlier 
criticism of the proposal focused on the city's plan to modify the 
resource through overloaded POST. 
</summary> 

<category scheme=" http://www.example.com/categories/RestfulNews" 
term="local" label="Local news" /> 

</entry> 

Service document 

This vaguely-named type of resource is just a grouping of collections. A typical move 
is to serve a single service document, listing all of your collections, as your service's 
"home page." A service document is an XML document written using a particular vo- 
cabulary, and its media type is application/atomserv+xml (see Example 9-6). 

Example 9-6 shows a representation of a typical service document. It describes three 
collections. One of them is a weblog called "RESTful news," which accepts a POST 
request if the representation is an Atom entry document like the one in Example 9-5 . 



276 | Chapter 9: The Building Blocks of Services 



The other two are personal photo albums, which accept a POST request if the repre- 
sentation is an image file. 

Example 9-6. A representation of a service document that describes three collections 

<?xml version="l.O" encoding=' utf-8' ?> 
<service xmlns=" http://purl.0rg/at0m/app#" 

xmlns:atom=" http://www.w3.org/2005/Atom"> 
<workspace> 
<atom:title>Weblogs</atom:title> 

collection href =" http://www.example.com/RestfulNews"> 
<atom:title>RESTful News</atom: title) 

categories href =" http://www.example.com/categories/RestfulNews" /> 
</collection> 
</workspace> 

<workspace> 
<atom:title>Photo galleries</atom:title> 
<collection 

href =" http://www.example.com/samruby/photos" > 
<atom:title>Sam' s photos</atom:title> 
<accept>image/*</accept> 

<categories href =" http://www.example.com/categories/samruby- photo" /> 
</collection> 

<collection 

href = " http : / /www . example . com/leonardr/ photos " > 
<atom:title>Leonard ' s photos</atom:title> 
<accept>image/*</accept> 

<categories href ="http: //www. example.com/categories/leonardr-photo" /> 
</collection> 
</workspace> 
</service> 

How do I know what kind of POST requests a collection will accept? From the 
accept tags. The accept tag works something like the HTTP Accept header, only in 
reverse. The Accept header is usually sent by the client with a GET request, to tell the 
server which representation formats the client understands. The accept tag is the APP 
server's way of telling the client which incoming representations a collection will accept 
as part of a POST request that creates a new member. 

My two photo gallery collections specify an accept of image/*. Those collections will 
only accept POST requests where the representation is an image. On the other hand, 
the RESTful News weblog doesn't specify an accept tag at all. The APP default is to 
assume that a collection only accepts POST requests when the representation is an 
Atom entry document (like the one in Example 9-5). The accept tag defines what the 
collections are for: the weblog is for textual data, and the photo collections are for 
images. 

The other important thing about a service document is the categories tag, which links 
to a "category document" resource. The category document says what categories are 
allowed. 



Prepackaged Control Flows | 277 



The APP doesn't say much about service documents. It specifies their representation 
format, and says that they must serve a representation in response to GET. It doesn't 
specify how service documents get on the server in the first place. If you write an APP 
application you can hardcode your service documents in advance, or you can make it 
possible to create new ones by POSTing to some new resource not covered by the APP. 
You can expose them as static files, or you can make them respond to PUT and DE- 
LETE. It's up to you. 



As you can see from Example 9-6, a service document's representation 
doesn't just describe collections: it groups collections into workspaces. 
" When I wrote that representation I put the weblog in a workspace of its 
own, and grouped the photo galleries into a second workspace. The APP 
standard devotes some time to workspaces, but I'm going to pass over 
them, because the APP doesn't define workspaces as resources. They 
don't have their own URIs, and they only exist as elements in the rep- 
resentation of a service document. You can expose workspaces as re- 
sources if you want. The APP doesn't prohibit it, but it doesn't tell you 
how to do it, either. 




Category documents 

APP members (which correspond to Atom elements) can be put into categories. In 
Chapter 7, I represented a bookmark's tags with Atom categories. The Atom entry 
described in Example 9-5 put the entry into a category called "local." Where did that 
category come from? Who says which categories exist for a given collection? This is the 
last big question the APP answers. 

The Atom entry document in Example 9-5 gave its category a "scheme" of http:// 
www.example.com/categories/RestfulNews. The representation of the RESTful News 
collection, in the service document, gave that same URI in its categories tag. That URI 
points to the final APP resource: a category document (see Example 9-7). A category 
document lists the category vocabulary for a particular APP collection. Its media type 
is application/atomcat+xml. 

Example 9-7 shows a representation of the category document for the collection 
"RESTful News." This category document defines three categories: "local," "interna- 
tional," and "lighterside," which can be referenced in Atom entry entities like the one 
in Example 9-5. 

Example 9-7. A representation of a category document 

<?xml version="l.O" ?> 
<app: categories 

xmlns : app="http : // purl . org/atom/app#" 

xmlns=" http: //www. w3.org/2005/Atom" 

scheme=" http: //www. example.com/categories/RestfulNews" 

fixed="no"> 
<category term="local" label="Local news"/> 



278 I Chapter 9: The Building Blocks of Services 



<category term="international" label="International news"/> 
<category term="lighterside" label="The lighter side of REST"/> 
</app:categories> 

The scheme is not fixed, meaning that it's OK to publish members to the collection 
even if they belong to categories not listed in this document. This document might be 
used in an end-user application to show a selectable list of categories for a new "RESTful 
news" story. 

As with service documents, the APP defines the representation format for a category 
document, but says nothing about how category documents are created, modified, or 
destroyed. It only defines GET on the category document resource. Any other opera- 
tions (like automatically modifying the category document when someone files an entry 
under a new category) are up to you to define. 

Binary documents as APP members 

There's one important wrinkle I've glossed over. It has to do with the "photo gallery" 
collections I described in Example 9-6. I said earlier that a client can create a new 
member in a photo gallery by POSTing an image file to the collection. But an image file 
can't go into an Atom feed: it's a binary document. What exactly happens when a client 
POSTs a binary document to an APP collection? What's in those photo galleries, really? 

Remember that a resource can have more than one representation. Each photo I upload 
to a photo collection has two representations. One representation is the binary photo, 
and the other is an XML document containing metadata. The XML document is an 
Atom entry, the same as the news item in Example 9-5, and that's the data that shows 
up in the Atom feed. 

Here's an example. I POST a JPEG file to my "photo gallery" collection, like so: 

POST /leonardr/photos HTTP/l.l 
Host: www.example.com 
Content-type: image/jpeg 
Content-length: 62811 
Slug: A picture of my guinea pig 

[DPEG file goes here] 

The Slug is a custom HTTP header defined by the APP, which lets me specify a title for 
the picture while uploading it. The slug can show up in several pieces of resource state, 
as you'll see in a bit. 

The HTTP response comes back as I described it in "Members" earlier in this chapter. 
The response code is 201 and the Location header gives me theURI of the newly created 
APP member. 

201 Created 

Location : http : //www. example . com/leonardr/photos/my-guinea-pig . atom 



Prepackaged Control Flows | 279 



But what's at the other end of the URI? Not the JPEG file I uploaded, but an Atom entry 
document describing and linking to that file: 

<![CDATA[ <?xml version="l.O" encoding="utf-8"?> 
<entry> 

<title>A picture of my guinea pig</title> 
<updated>2007-01-24Tll : 52 : 29Z</updated> 
<id>urn:flef2e50-8ec8-0129-bla7-003065546fl8</id> 
<summaryx/summary> 

<link rel="edit-media" type="image/jpeg" 

href ="http: //www. example. com/leonardr/photos/my-guinea-pig. jpg" /> 
</entry> 

The actual JPEG I uploaded is at the other end of that link. I can GET it, of course, 
and I can PUT to it to overwrite it with another image. My POST created a new "mem- 
ber" resource, and my JPEG is a representation of some of its resource state. But there's 
also this other representation of resource state: the metadata. These other elements of 
resource state include: 

• The title, which I chose (the server decided to use my Slug as the title) and can 
change later. 

• The summary, which starts out blank but I can change. 

• The "last update" time, which I sort of chose but can't change arbitrarily. 

• The URI to the image representation, which the server chose for me based on my 
Slug. 

• The unique ID, which the server chose without consulting me at all. 

This metadata document can be included in an Atom feed: I'll see it in the representa- 
tion of the "photo gallery" collection. I can also modify this document and PUT it back 
to http://www.example.com/leonardr/photos/20070124-l.atom to change the resource 
state. I can specify myself as the author, add categories, change the title, and so on. If 
I get tired of having this member in the collection, I can delete it by sending a DELETE 
request to either of its URIs. 

That's how the APP handles photos and other binary data as collection members. It 
splits the representation of the resource into two parts: the binary part that can't go 
into an Atom feed and the metadata part that can. This works because the metadata of 
publishing (categories, summary, and so on) applies to photos and movies just as easily 
as to news articles and weblog entries. 

*• . I If you read the APP standard (which you should, since this section 
doesn't cover everything), you'll see that it describes this behavior in 
f /; terms of two different resources: a "Media Link Entry," whose repre- 
sentation is an Atom document, and a "Media Resource," whose rep- 
resentation is a binary file. I've described one resource that has two 
representations. The difference is purely philosophical and has no effect 
on the actual HTTP requests and responses. 



280 | Chapter 9: The Building Blocks of Services 



Summary 

That's a fairly involved workflow, and I haven't even covered everything that the APP 
specifies, but the APP is just a well-thought-out way of handling a common web service 
problem: the list/feed/collection that keeps having items/elements/members added to 
it. If your problem fits this domain, it's easier to use the APP design — and get the 
benefits of existing client support — than to reinvent something similar (see Table 9-1). 

Table 9-1. APP resources and their methods 





GET 


POST 


PUT 


DELETE 


Service document 


Return a representation 
(XML) 


Undefined 


Undefined 


Undefined 


Category docu- 
ment 


Return a representation 
(XML) 


Undefined 


Undefined 


Undefined 


Collection 


Return a representation 
(Atom feed) 


Create a new member 


Undefined 


Undefined 


Member 


Return therepresentation 
identified by this URI. 
(This is usually an Atom 
entry document, but it 
might be a binary file.) 


Undefined 


Update the representa- 
tion identified bythisURI 


Delete the member 



GData 

I said earlier that the Atom Publishing Protocol defines only a few resources and only 
a few operations on those resources. It leaves a lot of space open for extension. One 
extension is Google's GData (http://code.google.com/apis/gdata), which adds a new 
kind of resource and some extras like an authorization mechanism. As of the time of 
writing, the Google properties Blogger, Google Calendar, Google Code Search, and 
Google Spreadsheets all expose RESTful web service interfaces. In fact, all four expose 
the same interface: the Atom Publishing Protocol with the GData extensions. 

Unless you work for Google, you probably won't create any services that expose the 
precise GData interface, but you may encounter GData from the client side. It's also 
useful to see how the APP can be extended to handle common cases. See how Google 
used the APP as a building block, and you'll see how you can do the same thing. 

Querying collections 

The biggest change GData makes is to expose a new kind of resource: the list of search 
results. The APP says what happens when you send a GET request to a collection's 
URI. You get a representation of some of the members in the collection. The APP 
doesn't say anything about finding specific subsets of the collection: finding members 
older than a certain date, written by a certain author, or filed under a certain category. 



Prepackaged Control Flows | 281 



It doesn't specify how to do full-text search of a member's text fields. GData fills in 
these blanks. 

GData takes every APP collection and exposes an infinite number of additional resour- 
ces that slice it in various ways. Think back to the "RESTful News" APP collection I 
showed in Example 9-2. The URI to that collection was http://www.example.com/Rest- 
fulNews. If that collection were exposed through a GData interface, rather than just an 
APP interface, the following URIs would also work: 

• http:/ '/www. example. com/RestfulNews?q=stadium: A subcollection of the members 
where the content contains the word "stadium." 

• http://www.example.eom/RestfulNews/-/local: A subcollection of the members 
categorized as "local." 

• http://www.example.com/RestfulNews?author=Tom%20Servo&max-results=50: 
At most 50 of the members where the author is "Tom Servo." 

Those are just three of the search possibilities GData exposes. (For a complete list, see 
the GData developer's guide (http://code.google.com/apis/gdata/reference.html). Note 
that not all GData applications implement all query mechanisms.) Search results are 
usually represented as Atom feeds. The feed contains a entry element for every member 
of the collection that matched the query. It also contains OpenSearch elements (q.v.) 
that specify how many members matched the query, and how many members fit on a 
page of search results. 

Data extensions 

I mentioned earlier that an Atom feed can contain markup from arbitrary other XML 
namespaces. In fact, I just said that GData search results include elements from the 
OpenSearch namespace. GData also defines a number of new XML entities in its own 
"gd" namespace, for representing domain-specific data from the Google web services. 

Consider an event in the Google Calendar service. The collection is someone's calendar 
and the member is the event itself. This member probably has the typical Atom fields: 
an author, a summary, a "last updated" date. But it's also going to have calendar- 
specific data. When does the event take place? Where will it happen? Is it a one-time 
event or does it recur? 

Google Calendar's GData API puts this data in its Atom feeds, using tags like gd : when, 
gd:who, and gd: recurrence. If the client understands Google Calendar's extensions it 
can act as a calendar client. If it only understands the APP, it can act as a general APP 
client. If it only understands the basic Atom feed format, it can treat the list of events 
as an Atom feed. 



282 | Chapter 9: The Building Blocks of Services 



POST Once Exactly 

POST requests are the fly in the ointment that is reliable HTTP. GET, PUT, and DE- 
LETE requests can be resent if they didn't go through the first time, because of the 
restrictions HTTP places on those methods. GET requests have no serious side effects, 
and PUT and DELETE have the same effect on resource state whether they're sent once 
or many times. But a POST request can do anything at all, and sending a POST request 
twice will probably have a different effect from sending it once. Of course, if a service 
committed to accepting only POST requests whose actions were safe or idempotent, it 
would be easy to make reliable HTTP requests to that service. 

POST Once Exactly (POE) is a way of making HTTP POST idempotent, like PUT and 
DELETE. If a resource supports Post Once Exactly, then it will only respond success- 
fully to POST once over its entire lifetime. All subsequent POST requests will give a 
response code of 405 ("Method Not Allowed"). A POE resource is a one-off resource 
exposed for the purpose of handling a single POST request. 




POE was defined by Mark Nottingham in an IETF draft that expired in 
2005. 1 think POE was a little ahead of its time, and if real services start 
■*,< implementing it, there could be another draft. 

You can see the original standard at http://www.mnot.net/drafts/draft- 
nottingham-http-poe-00.txt. 



Think of a "weblog" resource that responds to POST by creating a new weblog entry. 
How would we change this design so that no resource responds to POST more than 
once? Clearly the weblog can't expose POST anymore, or there could only ever be one 
weblog entry. Here's how POE does it. The client sends a GET or HEAD request to the 
"weblog" resource, and the response includes the special POE header: 

HEAD /weblogs/myweblog HTTP/l.l 
Host: www.example.com 
POE: 1 

The response contains the URI to a POE resource that hasn't yet been POSTed to. This 
URI is nothing more than a unique ID for a future POST request. It probably doesn't 
even exist on the server. Remember that GET is a safe operation, so the original GET 
request couldn't have changed any server state. 
200 OK 

POE -Links: /weblogs/myweblog/entry-factory-104a4ed 

POE and POE- Links are custom HTTP headers defined by the POE draft. POE just tells 
the server that the client is expecting a link to a POE resource. POE -Links gives one or 
more links to POE resources. At this point the client can POST a representation of its 
new weblog entry to /weblogs/rnyweblog/entry-factory-104a4ed. After the POST goes 
through, that URI will start responding to POST with a response code of 405 ("Oper- 
ation Not Supported"). If the client isn't sure whether or not the POST request went 



Prepackaged Control Flows | 283 



through, it can safely resend. There's no possiblity that the second POST will create a 
second weblog entry. POST has been rendered idempotent. 

The nice thing about Post Once Exactly is that it works with overloaded POST. Even 
if you're using POST in a way that totally violates the Resource-Oriented Architecture, 
your clients can use HTTP as a reliable protocol if you expose the overloaded POST 
operations through POE. 

An alternative to making POST idempotent is to get rid of POST altogether. Remember, 
POST is only necessary when the client doesn't know which URI it should PUT to. 
POE works by generating a unique ID for each of the client's POST operations. If you 
allow clients to generate their own unique IDs, they can use PUT instead. You can get 
the benefits of POE without exposing POST at all. You just need to make sure that two 
clients will never generate the same ID. 

Hypermedia Technologies 

There are two kinds of hypermedia: links and forms. A link is a connection between 
the current resource and some target resource, identified by its URI. Less formally, a 
link is any URI found in the body of a representation. Even JSON and plain text are 
hypermedia formats of a sort, since they can contain URIs in their text. But throughout 
this book when I say "hypermedia format," I mean a format with some kind of struc- 
tured support for links and forms. 

There are two kinds of forms. The simplest kind I'll call application forms, because they 
show the client how to manipulate application state. An application form is a way of 
handling resources whose names follow a pattern: it basically acts as a link with more 
than one destination. A search engine doesn't link to every search you might possibly 
make: it gives you a form with a space for you to type in your search query. When you 
submit the form, your browser constructs a URI from what you typed into the form 
(say, http-.llwwvj. google. coml sear ch?q=]elly fish), and makes a GET request to that URI. 
The application form lets one resource link to an infinite number of others, without 
requiring an infinitely large representation. 

The second kind of form I'll call resource forms, because they show the client how to 
format a representation that modifies the state of a resource. GET and DELETE re- 
quests don't need representations, of course, but POST and PUT requests often do. 
Resource forms say what the client's POST and PUT representations should look like. 

Links and application forms implement what I call connectedness, and what the Field- 
ing thesis calls "hypermedia as the engine of application state." The client is in charge 
of the application state, but the server can send links and forms that suggest possible 
next states. By contrast, a resource form is a guide to changing the resource state, which 
is ultimately kept on the server. 



284 | Chapter 9: The Building Blocks of Services 



I cover four hypermedia technologies in this section. As of the time of writing, XHTML 
4 is the only hypermedia technology in active use. But this is a time of rapid change, 
thanks in part to growing awareness of RESTful web services. XHTML 5 is certain to 
be widely used once it's finally released. My guess is that URI Templates will also catch 
on, whether or not they're incorporated into XHTML 5. WADL may catch on, or it 
may be supplanted by a combination of XHTML 5 and micro formats. 

URI Templates 

URI Templates (currently an Internet Draft (http://www.ietf.org/internet-drafts/draft- 
gregorio-uritemplate-OO.txt)) are a technology that makes simple resource forms look 
like links. I've used URI Template syntax whenever I want to show you an infinite 
variety of similar URIs. There was this example from Chapter 3, when I was showing 
you the resources exposed by Amazon's S3 service: 

https : //s3 . amazonaws . com/{ name-of-bucket} /{name-of -object } 

That string is not a valid URI, because curly brackets aren't valid in URIs, but it is a 
valid URI Template. The substring {name-of-bucket} is a blank to be filled in, a place- 
holder to be replaced with the value of the variable name-of-bucket. There are an infinite 
number of URIs lurking in that one template, including https://s3.amazonaws.com/ 
bucketl/objectl, https://s3.amazonaws.com/my-other-bucket/subdir/SomeObject.avi, 
and so on. 

URI templating gives us a precise way to play fill-in-the-blanks with URIs. Without 
URI Templates, a client must rely on preprogrammed URI construction rules based on 
English descriptions like https://s3.amazonaws.com/, and then the bucket name. 

URI Templates are not a data format, but any data format can improve its hypermedia 
capabilities by allowing them. There is currently a proposal to support URI Templates 
in XHTML 5, and WADL supports them already. 

XHTML 4 

HTML is the most successful hypermedia format of all time, but its success on the 
human web has typecast it as sloppy, and sent practitioners running for the more 
structured XML. The compromise standard is XHTML, an XML vocabulary for de- 
scribing documents which uses the same tags and attributes found in HTML. Since it's 
basically the same as HTML, XHTML has a powerful set of hypermedia features, 
though its forms are somewhat anemic. 

XHTML 4 links 

A number of HTML tags can be used to make hypertext links (consider img, for exam- 
ple), but the two main ones are link and a. A link tag shows up in the document's 
head, and connects the document to some resource. The link tag contains no text or 



Hypermedia Technologies | 285 



other tags: it applies to the entire document. An a tag shows up in the document's 
body. It can contain text and other tags, and it links its contents (not the document as 
a whole) to another resource (see Example 9-8) . 

Example 9-8. An XHTML 4 document with some links 
<!D0CTYPE html 

PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
"http://www.w3.org/TR/xhtmll/DTD/xhtmll-strict.dtd"> 
<html xmlns="http://www. w3.org/l999/xhtml" xml:lang="en"> 
<head> 

<link rel="alternate" type="application/atom+xml" href="atom.xml"> 
-clink rel="stylesheet" href="display.css"> 
</head> 

<body> 
<p> 

Have you read 

<a href = "Great- Expectations. html "><i>Great Expectations</ix/a>? 
</p> 
</body> 
</html> 

Example 9-8 shows a simple HTML document that contains both sorts of hyperlinks. 
There are two links that use link to relate the document as a whole to other URIs, and 
there's one link that uses a to relate part of the document (the italicized phrase "Great 
Expectations") to another URL 

The three important attributes of link and a tags are href, rel, and rev. The href at- 
tribute is the most important: it gives the URI of the resource that's being linked to. If 
you don't have an href attribute, you don't have a hyperlink. 

The rel attribute adds semantics that explain the foreign URLs relationship to this 
document. I mentioned this attribute earlier when I was talking about microformats. 
In Example 9-8, the relationship of the URI atom.xml to this document is "alternate". 
The relationship of the URI display, ess to this document is "stylesheet". These par- 
ticular values for rel are among the 15 defined in the HTML 4 standard. The value 
"alternate" means that the linked URI is an alternate representation of the resource this 
document represents. The value "stylesheet" means that the linked URI contains in- 
structions on how to format this document for display. Microformats often define ad- 
ditional values for rel. The rel-nofollow microformat defines the relationship "nofol- 
low", to show that a document doesn't trust the resource it's linking to. 

The rev attribute is the exact opposite of rel: it explains the relationship of this docu- 
ment to the foreign URI. The VoteLinks microformat lets you express your opinion of 
a URI by setting rev to "vote-for" or "vote-against". In this case, the foreign URI prob- 
ably has no relationship to you, but you have a relationship to it. 

A simple example illustrates the difference between rel and rev. Here's an HTML 
snippet of a user's home page, which contains two links to his father's home page. 



286 I Chapter 9: The Building Blocks of Services 



<a rel="parent" href="/Dad">My father</a> 
<a rev="child" href="/Dad">My father</a> 

XHTML 4 forms 

These are the forms that drive the human web. You might not have known about the 
rel and rev attributes, but if you've done any web programming, you should be familiar 
with the hypermedia capabilities of XHTML forms. 

To recap what you might already know: HTML forms are described with the form tag. 
A form tag has a method attribute, which names the HTTP method the client should use 
when submitting the form. It has an action attribute, which gives the (base) URI of the 
resource the form is accessing. It also has an enctype attribute, which gives the media 
type of any representation the client is supposed to send along with the request. 

A form tag can contain form elements: children like input and select tags. These show 
up in web browsers as GUI elements: text inputs, checkboxes, buttons, and the like. 
In application forms, the values entered into the form elements are used to construct 
the ultimate destination of a GET request. Here's an application form I just made up: 
an interface to a search engine. 

<form method="GET" action="http: //search. example.com/search"> 

<input name="query" type="text" /> 

<input type="submit" /> 
</form> 

Since this is an application form, it's not designed to operate on any particular resource. 
The point of the form is to use the URI in the action as a jumping-off point to an infinity 
of resources with user-generated URIs: http://search.example.com/search?q=jellyfish, 
http://search.example.com/search?q=chocolate, and so on. 

A resource form in HTML 4 identifies one particular resource, and it specifies an action 
of POST. The form elements are used to build up a representation to be sent along with 
the POST request. Here's a resource form I just made up: an interface to a file upload 
script. 

<form method="POST" action= "http://files.example.com/dir/subdir/" 
enctype="multipart/form-data"> 

<input type="text" name="description" /> 

<input type="file" name="newfile" /> 
</form> 

This form is designed to manipulate resource state, to create a new "file" resource as a 
subordinate resource of the "directory" resource at http://files.example.com/dir/sub- 
dirl '. The representation format is a "multipart/form-data" document that contains a 
textual description and a (possibly binary) file. 



Hypermedia Technologies | 287 



Shortcomings of XHTML 4 

HTML 4's hypermedia features are obviously good enough to give us the human web 
we enjoy today, but they're not good enough for web services. I have five major prob- 
lems with HTML's forms. 

1. Application forms are limited in the URIs they can express. You're limited to URIs 
that take a base URI and then tack on some key- value pairs. With an HTML ap- 
plication form you can "link" to http://search.example.com/search?q=jellyfish, but 
not http://search.example.com/search/jellyfish. The variables must go into the URLs 
query string as key-value pairs. 

2. Resource forms in HTML 4 are limited to using HTTP POST. There's no way to 
use a form to tell a client to send a DELETE request, or to show a client what the 
representation of a PUT request should look like. The human web, which runs on 
HTML forms, has a different uniform interface from web services as a whole. It 
uses GET for safe operations, and overloaded POST for everything else. If you want 
to get HTTP's uniform interface with HTML 4 forms, you'll need to simulate PUT 
and DELETE with overloaded POST (see "Faking PUT and DELETE" in Chap- 
ter 8 for the standard way) . 

3 . There's no way to use an HTML form to describe the HTTP headers a client should 
send along with its request. You can't define a form entity and say "the value of 
this entity goes into the HTTP request header X-Hy- Header . " I generally don't think 
services should require this of their clients, but sometimes it's necessary. The Atom 
Publishing Protocol defines a special request header (Slug, mentioned above) for 
POST requests that create a new member in a collection. The APP designers defined 
a new header, instead of requiring that this data go into the entity-body, because 
the entity-body might be a binary file. 

4. You can't use an HTML form to specify a representation more complicated than 
a set of key- value pairs. All the form elements are designed to be turned into key- 
value pairs, except for the "file" element, which doesn't help much. The HTML 
standard defines two content types for form representations: application/x-www- 
form-urlencoded, which is for key-value pairs (I covered it in "Form-encoding" in 
Chapter 6) ; and multipart/ form- data, which is for a combination of key-value pairs 
and uploaded files. 

You can specify any content type you want in enctype, just as you can put anything 
you want in a tag's class and rel attributes. So you can tell the client it should 
POST an XML file by setting a form's enctype to application/xml. But there's no 
way of conveying what should go into that XML file, unless it happens to be an 
XML representation of a bunch of key-value pairs. You can't nest form elements, 
or define new ones that represent data structures more complex than key-value 
pairs. (You can do a little better if the XML vocabulary you're using has its own 
media type, like application/atom+xml or application/rdf+xml.) 



288 | Chapter 9: The Building Blocks of Services 



5. As I mentioned in "Link the Resources to Each Other" in Chapter 5, you can't 
define a repeating field in an HTML form. You can define the same field twice, or 
ten times, but eventually you'll have to stop. There's no way to tell the client: "you 
can specify as many values as you want for this key- value pair. " 

XHTML 5 

HTML 5 solves many of the problems that turn up when you try to use HTML on the 
programmable web. The main problem with HTML 5 is the timetable. The official 
estimate has HTML 5 being adopted as a W3C Proposed Recommendation in late 2008. 
More conservative estimates push that date all the way to 2022. Either way, HTML 5 
won't be a standard by the time this book is published. That's not really the issue, 
though. The issue is when real clients will start supporting the HTML 5 features I 
describe below. Until they do, if you use the features of HTML 5, your clients will have 
to write custom code to interpret them. 

HTML 5 forms support all four basic methods of HTTP's uniform interface: GET, 
POST, PUT, and DELETE. I took advantage of this when designing my map applica- 
tion, if you'll recall Example 6-3. This is the easiest HTML 5 feature to support today, 
especially since (as I'll show in Chapter 11) most web browsers can already make PUT 
and DELETE requests. 

There's a proposal (not yet incorporated into HTML 5; see http://blog.welldesigne 
durls.org/2007/01/ll/proposing-uri-templates-for-webforms-2/) that would allow 
forms to use URI Templates. Under this proposal, an application form can have its 
template attribute (not its action attribute) be a URI Template like http : / / search . exam 
ple.com/search/{q}. It could then define q as a text field within the form. This would 
let you use an application form to "link" to http://search.example.com/search/jelly 
fish. 

HTML 4 forms can specify more than one form element with the same name. This lets 
clients know they can submit the same key with 2 or 10 values: as many values as there 
are form elements. HTML 5 forms support the "repetition model," a way of telling the 
client it's allowed to submit the same key as many times as it wants. I used a simple 
repetition block in Example 5-11. 

Finally, HTML 5 defines two new ways of serializing key-value pairs into representa- 
tions: as plain text, or using a newly defined XML vocabulary. The content type for the 
latter is application/x-www-form+xml. This is not as big an advance as you might think. 
Form entities like input are still ways of getting data in the form of key-value pairs. 
These new serialization formats are just new ways of representing those key- value pairs. 
There's still no way to show the client how to format a more complicated representa- 
tion, unless the client can figure out the format from just the content type. 



Hypermedia Technologies | 289 



Where Am I Getting All This? 

The Web Hypertext Application Technology Working Group (WHATWG) is devel- 
oping the standards that will become HTML 5. The overarching standard is the Web 
Applications 1.0 standard (http://www.whatwg.org/specs/web-apps/current-workf), but 
all the changes to HTML's hypermedia capabilities are in the Web Forms 2.0 stand- 
ard (http://www.whatwg.org/specs/web-forms/current-work/). That's the document that 
describes all of these features. 



WADL 

The Web Application Description Language is an XML vocabulary for expressing the 
behavior of HTTP resources (see the development site for the Java client (https:// 
wadl.dev.java.net/)) . It was named by analogy with the Web Service Description Lan- 
guage, a different XML vocabulary used to describe the SOAP -based RPC-style services 
that characterize Big Web Services. 

Look back to "Service document" earlier in this chapter where I describe the Atom 
Publishing Protocol's service documents. The representation of a service document is 
an XML document, written in a certain vocabulary, which describes a set of resources 
(APP collections) and the operations you're allowed to perform on those resources. 
WADL is a standard vocabulary that can do for any resource at all what APP service 
documents do for APP collection resources. 

You can provide a WADL file that describes every resource exposed by your service. 
This corresponds roughly to a WSDL file in a SOAP/WSDL service, and to the "site 
map" pages you see on the human web. Alternatively, you can embed a snippet of 
WADL in an XML representation of a particular resource, the way you might embed 
an HTML form in an HTML representation. The WADL snippet tells you how to 
manipulate the state of the resource. 

As I said way back in Chapter 2, WADL makes it easy to write clients for web services. 
A WADL description of a resource can stand in for any number of programming-lan- 
guage interfaces to that resource: all you need is a WADL client written in the appro- 
priate language. WADL abstracts away the details of HTTP requests, and the building 
and parsing of representations, without hiding HTTP's uniform interface. 

As of the time of writing, WADL is more talked about than used. There's a Java client 
implementation (http://wadl.dev.java.net/), a rudimentary Ruby client (http:// 
www.crummy.com/software/wadl.rbf), and that's about it. Most existing WADL files 
are bootleg descriptions of other peoples' RESTful and REST-RPC services. 

WADL does better than HTML 5 as a hypermedia format. It supports URI Templates 
and every HTTP method there is. A WADL file can also tell the client to populate certain 
HTTP headers when it makes a request. More importantly, WADL can describe rep- 
resentation formats that aren't just key-value pairs. You can specify the format of an 



290 | Chapter 9: The Building Blocks of Services 



XML representation by pointing to a schema definition. Then you can point out which 
parts of the document are most important by specifying key-value pairs where the 
"keys" are XPath statements. This is a small step, but an important one. With HTML 
you can only specify the format of an XML representation by giving it a different content 
type. 

Of course, the "small step" only applies to XML. You can use WADL to say that a 
certain resource serves or accepts a JSON document, but unless that JSON document 
happens to be a hash (key- value pairs again!), there's no way to specify what the JSON 
document ought to look like. This is a general problem which was solved in the XML 
world with schema definitions. It hasn't been solved for other formats. 

Describing a del.icio.us resource 

Example 9-9 shows a Ruby client for the del.icio.us web service based on Ruby's WADL 
library. It's a reprint of the code from "Clients Made Easy with WADL" in Chapter 2. 

Example 9-9. A Ruby/WADL client for del.icio.us 

#!/usr/bin/ruby 

# delicious-wadl-ruby.rb 
require 'wadl' 

if ARCV.size != 2 

puts "Usage: #{$0} [username] [password]" 

exit 
end 

username, password = ARCV 

# Load an application from the WADL file 

delicious = WADL: Application. from_wadl(open("delicious. wadl")) 

# Give authentication information to the application 
service = delicious. vl.with_basic_auth(username, password) 

begin 

# Find the "recent posts" functionality 
recent_posts = service. posts. recent 

# For every recent post... 

recent_posts. get. representation. each_by_param( ' post ' ) do | post | 
# Print its description and URI. 

puts "#{post.attributes[ 'description' ]}: #{post.attributes[ 'href ]}" 
end 

rescue WADL: : Faults: :AuthorizationRequired 

puts "Invalid authentication information!" 
end 

The code's very short but you can see what's happening, especially now that we're past 
Chapter 2 and I've shown you how resource-oriented services work. The del.icio.us 
web service exposes a resource that the WADL library identifies with vl. That resource 
has a subresource identified by posts. recent. If you recall the inner workings of 



Hypermedia Technologies | 291 



del.icio.us from Chapter 2, you'll recognize this as corresponding to the URI https:// 
api.del.icio.us/vl/posts/recent. When you tell the WADL library to make a GET request 
to that resource, you get back some kind of response object which includes an XML 
representation. Certain parts of this representation, the posts, are especially interest- 
ing, and I process them as XML elements, extracting their descriptions and hrefs. 

Let's look at the WADL file that makes this code possible. I've split it into three sections: 
resource definition, method definition, and representation definition. Example 9-10 
shows the resource definition. I've defined a nested set of WADL resources: vl inside 
posts inside recent. The recent WADL resource corresponds to the HTTP resource 
the del.icio.us API exposes at https://api.del.icio.us/vl/posts/recent. 

Example 9-10. WADL file for del.icio.us: the resource 
<?xml version="l.0"?> 

<!-- This is a partial bootleg WADL file for the del.icio.us API. --> 
opplication xmlns= " http : / /research . sun . com/wadl/ 2006/07" > 
<!-- The resource --> 

<resources base="https://api.del.icio.us/"> 
<doc xml:lang="en" title="The del.icio.us API vl"> 

Post or retrieve your bookmarks from the social networking website. 

Limit requests to one per second. 
</doc> 

<resource path="vl"> 

<param name="Authorization" style="header" required="true"> 
<doc xml:lang="en">All del.icio.us API calls must be authenticated 
using Basic HTTP auth.</doc> 

</param> 

<resource path="posts"> 
<resource path="recent"> 

<method href="#getRecentPosts" /> 
</resource> 

</resource> 
</resource> 
</resources> 

That HTTP resource exposes a single method of the uniform interface (GET), so I define 
a single WADL method inside the WADL resource. Rather than define the method 
inside the resource tag and clutter up Example 9-10, I've defined it by reference. I'll get 
to it next. 

Every del.icio.us API request must include an Authorization header that encodes your 
del.icio.us username and password using HTTP Basic Auth. I've represented this with 
a param tag that tells the client it must provide an Authorization header. The param tag 
is the equivalent of an HTML form element: it tells the client about a blank to be filled 
in.§ 



292 | Chapter 9: The Building Blocks of Services 



Example 9-11 shows the definition of the method getRecentPosts. A WADL method 
corresponds to a request you might make using HTTP's uniform interface. The id of 
the method can be anything, but its name is always the name of an HTTP method: here, 
"GET". The method definition models both the HTTP request and response. 

Example 9-11. WADL file for del.icio.us: the method 
< ! - - The method --> 

<method id="getRecentPosts" name="GET"> 
<doc xml:lang="en" title="Returns a list of the most recent posts." /> 

<request> 

<param name="tag" style="form"> 
<doc xml:lang="en" title="Filter by this tag." /> 

</param> 

<param name="count" style="form" default="l5"> 
<doc xml:lang="en" title="Number of items to retrieve. "> 

Maximum: 100 
</doc> 

</param> 
</request> 

<response> 

representation href="#postList" /> 

<fault id="AuthorizationRequired" status="40l" /> 
</response> 
</method> 

This particular request defines two more params: two more blanks to be filled in by the 
client. These are "query" params, which in a GET request means they'll be tacked onto 
the query string — just like elements in an HTML form would be. These param defini- 
tions make it possible for the WADL client to access URIs like https://api.del.icio.us/vl/ 
posts/recent?count=l 00 and https://api.dd.icio.us/vl/posts/recent?tag=rest&count=20. 

This WADL method defines an application form: not a way of manipulating resource 
state, but a pointer to possible new application states. This method tag tells the client 
about an infinite number of GET requests they can make to a set of related resources, 
without having to list infinitely many URIs. If this method corresponded to a PUT or 
POST request, its request might be a resource form, a way of manipulating resource 
state. Then it might describe a representation for you to send along with your request. 

The response does describe a representation: the response document you get back 
from del.icio.us when you make one of these GET requests. It also describes a possible 
fault condition: if you submit a bad Authorization header, you'll get a response code 
of 401 ("Unauthorized") instead of a representation. 



§ Marc Hadley, the primary author of the WADL standard, is working on more elegant ways of representing 
the need to authenticate. 



Hypermedia Technologies | 293 



Take a look at Example 9-12, which defines the representation. This is WADL's de- 
scription of the XML document you receive when you GET https://api.del.icio.us/vl/ 
posts/recent: a document like the one in Example 2-3. 

Example 9-12. WADL file for del.icio.us: the representation 
<!-- The representation --> 

•(representation id="postl_ist" mediaType="text/xml" element="posts"> 

<param name="post" path="/posts/post" repeating="true" /> 
</representation> 

</application> 

The WADL description gives the most important points about this document: its con- 
tent type is text/xml, and it's rooted at the posts tag. The param tag points out that the 
the posts tag has a number of interesting children: the post tags. The param's path 
attribute gives an XPath expression which the client can use on the XML document to 
fetch all the del.icio.us posts. My client's call to each_by_param( ' post ' ) runs that XPath 
expression against the document, and lets me operate on each matching element with- 
out having to know anything about XPath or the structure of the representation. 

There's no schema definition for this kind of XML representation: it's a very simple 
document and del.icio.us just assumes you can figure out the format. But for the sake 
of demonstration, let's pretend this representation has an XML Schema Definition 
(XSD) file. The URI of this imaginary definition is https://api.del.icio.us/vl/posts.xsd, 
and it defines the schema for the posts and post tags. In that fantasy situation, Exam- 
ple 9-13 shows how I might define the representation in terms of the schema file. 

Example 9-13. WADL file for delicious: the resource 
<?xml version="l.0"?> 

<!-- This is a partial bootleg WADL file for the del.icio.us API. --> 

opplication xmlns="http : / /research . sun . com/wadl/ 2006/07" 

xmlns : delicious= " https : //api . del . icio . us/vl/post s . xsd " > 

<grammars> 

•(include "https://api.del.icio.us/vl/posts.xsd" /> 
</grammars> 



•(representation id="postl_ist" mediaType="text/xml" element="delicious:posts" /> 
</application> 

I no longer need a param to say that this document is full of post tags. That information's 
in the XSD file. I just have to define the representation in terms of that file. I do this by 
referencing the XSD file in this WADL file's grammars, assigning it to the delicious: 
namespace, and scoping the representation's element attribute to that namespace. If 
the client is curious about what a delicious: posts tag might contain, it can check the 



294 | Chapter 9: The Building Blocks of Services 



XSD. Even though the XSD completely describes the representation format, I might 
define some param tags anyway to point out especially important parts of the document. 

Describing an APP collection 

That was a pretty simple example. I used an application form to describe an infinite set 
of related resources, each of which responds to GET by sending a simple XML docu- 
ment. But I can use WADL to describe the behavior of any resource that responds to 
the uniform interface. If a resource serves an XML representation, I can reach into that 
representation with param tags: show where the interesting bits of data are, and where 
the links to other resources can be found. 

Earlier I compared WADL files to the Atom Publishing Protocol's service documents. 
Both are XML vocabularies for describing resources. Service documents describe APP 
collections, and WADL documents describe any resource at all. You've seen how a 
service document describes a collection (Example 9-6). What would a WADL descrip- 
tion of the same resources look like? 

As it happens, the WADL standard gives just this example. Section A. 2 of the standard 
shows an APP service document and then a WADL description of the same resources. 
I'll present a simplified version of this idea here. 

The service document in Example 9-6 describes three Atom collections. One accepts 
new Atom entries via POST, and the other two accept image files. These collections 
are pretty similar. In an object-oriented system I might factor out the differences by 
defining a class hierarchy. I can do something similar in WADL. Instead of defining all 
three resources from scratch, I'm going to define two resource types. Then it'll be simple 
to define individual resources in terms of the types (see Example 9-14). 

Example 9-14. A WADL file for APP: resource types 
<?xml version="l.O"?> 

<!-- This is a description of two common types of resources that respond 
to the Atom Publishing Protocol. --> 

opplication xmlns= " http : / /research . sun . com/wadl/ 2006/07" 
xmlns : app= " http : //purl . org/atom/app" > 

<!-- An Atom collection accepts Atom entries via POST. --> 
<resource_type id="atom_collection " > 

<method href="#getCollection" /> 

<method href="#postNewAtomMember" /> 
</resource_type> 

<!-- An image collection accepts image files via POST. --> 
<resource_type id="image_collection"> 

<method href="#getCollection" /> 

<method href="#postNewImageMember" /> 
</resource_type> 



Hypermedia Technologies | 295 



There are my two resource types: the Atom collection and the image collection. These 
don't correspond to any specific resources: they're equivalent to classes in an object- 
oriented design. Both "classes" support a method identified as getCollection, but the 
Atom collection supports a method postNewAtomMember where the image collection 
supports postNewImageMember. Example 9-15 shows those three methods: 

Example 9-15. A WADL file for APP: methods 

<!-- Three possible operations on resources. --> 
<method name="GET" id="getCollection"> 

<response> 
representation href="#feed" /> 

</response> 
</method> 

<method name="P0ST" id="postNewAtomMember"> 

<request> 
representation href="#entry" /> 

</request> 
</method> 

<method name="P0ST" id="postNewImageMember"> 
<request> 

<representation id="image" mediaType="image/*" /> 
<param name="Slug" style="header" /> 
</request> 
</method> 

The getCollection WADL method is revealed as a GET operation that expects an Atom 
feed (to be described) as its representation. The postNewAtomMember method is a POST 
operation that sends an Atom entry (again, to be described) as its representation. The 
postNewImageMember method is also a POST operation, but the representation it sends 
is an image file, and it knows how to specify a value for the HTTP header Slug. 

Finally, Example 9-16 describes the two representations: Atom feeds and atom entries. 
I don't need to describe these representations in great detail because they're already 
described in the XML Schema Document for Atom: I can just reference the XSD file. 
But I'm free to annotate the XSD by defining param elements that tell a WADL client 
about the links between resources. 

Example 9-16. A WADL file for APP: the representations 

<!-- Two possible XML representations. --> 
•(representation id="feed" mediaType="application/atom+xml" 
element="atom:feed" /> 

•representation id="entry" mediaType="application/atom+xml" 
element="atom:entry" /> 

</application> 

lean make the file I just defined available on the Web: say, at http://www.example.com/ 
app-resource-types .wadl. Now it's a resource. I can use it in my services by referencing 



296 | Chapter 9: The Building Blocks of Services 



its URL So can anyone else. It's now possible to define certain APP collections in terms 
of these resource types. My three collections are defined in just a few lines in Exam- 
ple 9-17. 

Example 9-17. A WADL file for a set of APP collections 
<?xml version="l.0"?> 

<!-- This is a description of three "collection" resources that respond 
to the Atom Publishing Protocol. --> 

opplication xmlns= " http : / /research . sun . com/wadl/ 2006/07" 
xmlns : app= " http : //purl . org/atom/app" > 
<resources base="http : //www. example . com/" > 
<resource path="RESTfulNews" 

type=" http://www.example.eom/app-resource-types.wadl#atom_collection" /> 
<resource path="samruby/photos" 

type=" http://www.example.eom/app-resource-types.wadl#image_collection" /> 
<resource path="leonardr/photos" 

type=" http: //www. example.com/app-resource- types. wadl#image_collection"/> 
</resources> 
</application> 

The Atom Publishing Protocol is popular because it's such a general interface. The 
major differences between two APP services are described in the respective service 
documents. A generic APP client can read these documents and reprogram itself to act 
as a client for many different services. But there's an even more general interface: the 
uniform interface of HTTP. An APP service document uses a domain-specific XML 
vocabulary, but hypermedia formats like HTML and WADL can be used to describe 
any web service at all. Their clients can be even more general than APP clients. 

Hypermedia is how one service communicates the ways it differs from other services. 
If that intelligence is embedded in hypermedia, the programmer needs to hardwire less 
of it in code. More importantly, hypermedia gives you access to the link: the second 
most important web technology after the URL The potential of REST will not be fully 
exploited until web services start serving their representations as link-rich hypermedia 
instead of plain media. 

Is WADL evil? 

In Chapter 10 I'll talk about how WSDL turned SOAP from a simple XML envelope 
format to a name synonymous with the RPC style of web services. WSDL abstracts 
away the details of HTTP requests and responses, and replaces them with a model 
based on method calls in a programming language. Doesn't WADL do the exact same 
thing? Should we worry that WADL will do to plain-HTTP web services what WSDL 
did to SOAP web services: tie them to the RPC style in the name of client convenience? 

I think we're safe. WADL abstracts away the details of HTTP requests and responses, 
but — this is the key point — it doesn't add any new abstraction on top. Remember, 
REST isn't tied to HTTP. When you abstract HTTP away from a RESTful service, 
you've still got REST. A resource-oriented web service exposes resources that respond 



Hypermedia Technologies | 297 



to a uniform interface: that's REST. A WADL document describes resources that re- 
spond to a uniform interface: that's REST. A program that uses WADL creates objects 
that correspond to resources, and accesses them with method calls that embody a uni- 
form interface: that's REST. RESTfulness doesn't live in the protocol. It lives in the 
interface. 

About the worst you can do with WADL is hide the fact that a service responds to the 
uniform interface. I've deliberately not shown you how to do this, but you should be 
able to figure it out. You may need to do this if you're writing a WADL file for a web 
application or REST-RPC hybrid service that doesn't respect the uniform interface. 

I'm fairly sure that WADL itself won't tie HTTP to an RPC model, the way WSDL did 
to SOAP. But what about those push-button code generators, the ones that take your 
procedure-call-oriented code and turn it into a "web service" that only exposes one 
URI? WADL makes you define your resources, but what if tomorrow's generator creates 
a WADL file that only exposes a single "resource", the way an autogenerated WSDL 
file exposes a single "endpoint"? 

This is a real worry. Fortunately, WADL's history is different from WSDL's. WSDL 
was introduced at a time when SOAP was still officially associated with the RPC style. 
But WADL is being introduced as people are becoming aware of the advantages of 
REST, and it's marketed as a way to hide the details while keeping the RESTful inter- 
face. Hopefully, any tool developers who want to make their tools support WADL will 
also be interested in making their tools support RESTful design. 



298 | Chapter 9: The Building Blocks of Services 



CHAPTER 10 

The Resource-Oriented Architecture 

Versus Big Web Services 



Throughout this book I've focused on technologies and architectures that work with 
the grain of the Web. I've shown you how to arrange resources into services that are 
very powerful, but conceptually simple and accessible from a wide variety of standard 
clients. But I've hardly said a word about the technologies that most people think of 
when they think web services: SOAP, WSDL, and the WS-* stack. These technologies 
form a competing paradigm for web services: one that doesn't really work like the Web. 
Rather than letting these technologies claim the entire field of web services for them- 
selves, I've given them a not entirely kind, but fairly mild nickname: Big Web Services. 
In this chapter I'll compare the two paradigms. 

The web is based on resources, but Big Web Services don't expose resources. The Web 
is based on URIs and links, but a typical Big Web Service exposes one URI and zero 
links. The Web is based on HTTP, and Big Web Services hardly use HTTP's features 
at all. This isn't academic hair-splitting, because it means Big Web Services don't get 
the benefits of resource-oriented web services. They're not addressable, cacheable, or 
well connected, and they don't respect any uniform interface. (Many of them are state- 
less, though.) They're opaque, and understanding one doesn't help you understand the 
next one. In practice, they also tend to have interoperability problems when serving a 
variety of clients. 

In this chapter I apply the same analytical tools to Big Web Services as I've been using 
to explain the REST architectural style and the Resource-Oriented Architecture. I'll be 
covering a lot of ideas in just a few pages — there are already whole books about these 
technologies — but my goal is not to give you a complete introduction. I just want to 
show you how the two philosophies line up. I'll examine technologies like SOAP on a 
technical level, and not in terms of how they've been hyped or demonized. I'll focus on 
these specifications as they're widely deployed, and less on the unrealized potential of 
newer versions. 

The vision of Big Web Services has evolved over time and not all practitioners are up 
to date on the latest concepts. To make sure I get everybody, I'm going to take a 



299 



chronological approach to my analysis. I'll start with the original "publish, find, and 
bind" vision, move on to "secure, reliable transactions," and finally touch on more 
recent developments like the Enterprise Server Bus, Business Process Execution Lan- 
guage, and Service-Oriented Architecture. 

What Problems Are Big Web Services Trying to Solve? 

As I said, the Web is resource-oriented. To implement the RPC style atop it is to go 
against the grain of the Web. But the Web wasn't designed to support general-purpose 
distributed programming. Sometimes your application has a natural grain of its own, 
and going against that is problematic. 

Here's a concrete example that I'll come back to throughout this chapter: a service that 
sets up travel reservations. Booking a trip might require booking a flight, a hotel, and 
a rental car. These tasks are interrelated: getting a rental car and a seat on a flight may 
be of little use to the client if you can't find a hotel. Each task requires coordinating 
with external authorities to find the best deal: the airlines, the rental car companies, 
the hotels. Each of these external authorities may be a separate service, and dealing 
with them involves making commitments. You may be able to have the airline service 
hold a seat on a plane for five minutes while you try to line up the rest of the deal. You 
may need to make a hotel reservation that will bill the customer for the first night's stay 
whether or not they show up. These time-limited commitments represent shared state. 

The resource-oriented approach I advocate in this book is Turing-complete. It can 
model any application, even a complex one like a travel broker. If I implemented this 
travel broker as a set of resource-oriented services, I'd expose resources like "a five- 
minute hold on seat 24C." This would work, but there's probably little value in that 
kind of resource. I don't pretend to know what emergent properties might show up in 
a resource-oriented system like this, but it's not likely that someone would want to 
bookmark that resource's URI and pass it around. 

The travel agency service has a different grain than the rest of the Web. This doesn't 
mean that it can't be made into a successful web application. Nor does it imply that 
SOAP and related specifications are a better fit. But this is the main problem that Big 
Web Services are trying to solve: the design of process-oriented, brokered distributed 
services. For whatever reason, this kind of application tends to be more prevalent in 
businesses and government applications, and less prevalent in technical and academic 
areas. 

SOAP 

SOAP is the foundation on which the plethora of WS-* specifications is built. Despite 
the hype and antihype it's been subjected to, there's amazingly little to this specifica- 
tion. You can take any XML document (so long as it doesn't have a DOCTYPE or 



300 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



processing instructions) , wrap it in two little XML elements, and you have a valid SOAP 
document. For best results, though, the document's root element should be in a name- 
space. 

Here's an XML document: 

•chello-world xmns= "http: //example. com" /> 

Here's the same document, wrapped in a SOAP envelope: 

<soap : Envelope xmlns : soap="http : //schemas . xmlsoap . org/soap/envelope/" > 

<soap:Body> 
< hello-world xmns=" http: //example. com" /> 

</soap:Body> 
</soap:Envelope> 

The only catch is that the SOAP Envelope must have the same character encoding as 
the document it encloses. That's pretty much all there is to it. Wrapping an XML 
document in two extra elements is certainly not an unreasonable or onerous task, but 
it doesn't exactly solve all the world's problems either. 

Seem too simple? Here's a real-world example. In Example 1-8 1 showed you an elided 
version of a SOAP document you might submit to Google's web search service. Exam- 
ple 10-1 shows the whole document. 

Example 10-1. A SOAP envelope to be submitted to Google's SOAP search service 

<soap: Envelope xmlns :soap="http: //schemas. xmlsoap.org/soap/envelope/"> 
<soap:Body> 

<gs:doGoogleSearch xmlns :gs="urn:GoogleSearch"> 
<key>oooooooooooooooooooooooooooooooo</key> 
<q>REST book</q> 
<start>0</start> 
<maxResults>10</maxResults> 
<filter>true</filter> 
<restrict/> 

<safeSearch>false</safeSearch> 
<lr/> 

<ie>latinl</ie> 
<oe>latinl</oe> 
</gs : doCoogleSearch> 
</soap:Body> 
</soap:Envelope> 

This document describes a Call to the Remote Procedure gs : doGoogleSearch. All of the 
query parameters are neatly tucked into named elements. This example is fully func- 
tional, though if you POST it to Google you'll get back a fault document saying that 
the key is not valid. 

This style of encoding parameters to a remote function is sometimes called RPC/literal 
or Section 5 encoding. That's the section in the SOAP 1 . 1 specification that shows how 
to use SOAP for RPC. But over time, fashions change. Later versions of the specification 
made support of this encoding optional, and so it's now effectively deprecated. It was 



SOAP | 301 



largely replaced by an encoding called document/literal, and then by wrapped 
document/literal. Wrapped document/literal looks largely the same as section 5 en- 
coding, except that the parameters tend to be scoped to a namespace. 

One final note about body elements: the parameters may be annotated with data type 
information based on XML Schema Data Types. This annotation goes into attributes, 
and generally reduces the readability of the document. Instead of <ie>latinl</ie> you 
might see <ie xsi:type="xsd:string">latinl</ie>. Multiply that by the number of 
arguments in Example 10-1 and you may start to see why many recoil in horror when 
they hear "SOAP." 

In Chapter 1 1 said that HTTP and SOAP are just different ways of putting messages in 
envelopes. HTTP's main moving parts are the entity-body and the headers. With a 
SOAP element named Body, you might expect to also find a Header element. You'd be 
right. Anything that can go into the Body element — any namespaced document which 
has no DOCTYPE or processing instructions — can go into the Header. But while you 
tend to only find a single element inside the Body, the Header can contain any number 
of elements. Header elements also tend to be small. 

Recalling the terminology used in "HTTP: Documents in Envelopes" in Chapter 1, 
headers are like "stickers" on an envelope. SOAP headers tend to contain information 
about the data in the body, such as security and routing information. The same is true 
of HTTP headers. 

SOAP defines two attributes for header entities: actor and mustUnderstand. If you know 
in advance that your message is going to pass through intermediaries on the way to its 
destination, you can identify (via a URI) the actor that's the target of any particular 
header. The mustUnderstand attribute is used to impose restrictions on those interme- 
diaries (or on the final destination). If the actor doesn't understand a header addressed 
to it, and mustUnderstand is true, it must reject the message — even if it thinks it could 
handle the message otherwise. An example of this would be a header associated with 
a two-phase commit operation. If the destination doesn't understand two-phase com- 
mit, you don't want the operation to proceed. 

Beyond that, there isn't much to SOAP. Requests and responses have the same format, 
similar to HTTP. There's a separate format for a SOAP Fault, used to signify an error 
condition. Right now the only thing that can go into a SOAP document is an XML 
document. There have been a few attempts to define mechanisms for attaching binary 
data to messages, but no clear winner has emerged. 

Given this fairly simple protocol, what's the basis for the hype and controversy? SOAP 
is mainly infamous for the technologies built on top of it, and I'll cover those next. It 
does have one alleged benefit of its own: transport independence. The headers are inside 
the message, which means they're independent of the protocol used to transport the 
message. You don't have to send a SOAP envelope inside an HTTP envelope. You can 
send it over email, instant messaging, raw TCP, or any other protocol. In practice, this 
feature is rarely used. There's been some limited public use of SMTP transports, and 



302 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



some use of JMS transports behind the corporate firewall, but the overwhelming ma- 
jority of SOAP traffic is over HTTP. 

The Resource-Oriented Alternative 

SOAP is almost always sent over HTTP, but SOAP toolkits make little use of HTTP 
status codes, and tend to coerce all operations into POST methods. This is not tech- 
nically disallowed by the REST architectural style, but it's a degenerate sort of RESTful 
architecture that doesn't get any of the benefits REST is supposed to provide. Most 
SOAP services support multiple operations on diverse data, all mediated through POST 
on a single URI. This isn't resource-oriented: it's RPC-style. 

The single most important change you can make is to split your service into resources: 
identify every "thing" in your service with a separate URI. Pretty much every SOAP 
toolkit in existence provides access to this information, so use it! Put the object reference 
up front. Such usages may not feel idiomatic at first, but if you stop and think about 
it, this is what you'd expect to be doing if SOAP were really a Simple Object Access 
Protocol. It's the difference between object-oriented programming in a function-ori- 
ented language like C: 

my_f unction (object, argument); 

and in an object-oriented language like C++: 
object->my_method(argument) ; 

When you move the scoping information outside the parentheses (or, in this case, the 
Envelope), you'll soon find yourself identifying large numbers of resources with com- 
mon functionality. You'll want to refactor your logic to exploit these commonalities. 

The next most important change has to do with the object-oriented concept of poly- 
morphism. You should try to make objects of different types respond to method calls 
with the same name. In the world of the Web, this means (at a minimum) supporting 
HTTP's GET method. Why is this important? Think about a programming language's 
standard library. Pretty much every object-oriented language defines a standard class 
hierarchy, and at its root you find an Object class which defines a toString method. 
The details are different for every language, but the result is always the same: every 
object has a method that provides a canonical representation of the object. The GET 
method provides a similar function for HTTP resources. 

Once you do this, you'll inevitably notice that the GET method is used more heavily than 
all the other methods you have provided. Combined. And by a wide margin. That's 
where conditional GET and caching come in. Implement these standard features of 
HTTP, make your representations cacheable, and you make your application more 
scalable. That has direct and tangible economic benefits. 

Once you've done these three simple things, you may find yourself wanting more. 
Chapter 8 is full of advice on these topics. 



SOAP | 303 



WSDL 



SOAP provides an envelope for your messages, but little else. Beyond section 5 (which 
is falling out of favor), SOAP doesn't constrain the structure of the message one bit. 
Many environments, especially those that depend on static typing, need a bit more 
definition up front. That's where WSDL comes in. 

I'm going to illustrate the concepts behind WSDL using the weblogs.com SOAP 1.1 
interface (http://www.soapware.org/weblogsCom). I chose it because it's pretty much 
the simplest deployed SOAP interface out there. Any service you encounter or write 
will undoubtedly be more complicated, but the basic steps are the same. 

The weblogs . com interface exposes a single RPC-style function called ping. The function 
takes two arguments, both strings, and returns a pingResult structure. This custom 
structure contains two elements: f lerror, a Boolean, and message, a string. Strings and 
Booleans are standard primitive data types, but to use a pingResult I need to define it 
as an XML Schema complexType. I'll do this within the types element of my WSDL file 
in Example 10-2. 

Example 10-2. XML Schema definition of the pingResult struct 
<types> 

<s: schema targetNamespace="uri:weblogscom"> 
<s: complexType name=" pingResult "> 
<s:sequence> 

<s:element minOccurs="l" maxOccurs="l" name="flerror" type="s:boolean"/> 
<s: element minOccurs="l" maxOccurs="l" name="message" type="s: string" /> 
</s:sequence> 
</s: complexType) 
</s:schema> 
</types> 

Now that I've defined the custom type, I'll move on to defining the messages that can 
be sent between client and server. There are two messages here: the ping request and 
the ping response. The request has two parts, and the response has only one (see 
Example 10-3). 

Example 10-3. WSDL definitions of the ping messages 

<message name="pingRequest"> 

<part name="weblogname" type="s:string"/> 

<part name="weblogurl" type="s:string"/> 
</message> 

<message name="pingResponse"> 

<part name="result" type="tns:pingResult"/> 
</message> 

Now I can use these messages in the definition of an WSDL operation, which is defined 
as a part of a port type. A port is simply a collection of operations. A programming 
language would refer to this as a library, a module, or a class. But this is the world of 



304 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



messaging, so the connection points are called ports, and an abstract definition of a 
port is called a port type. In this case, I'm defining a port type that supports a single 
operation: ping (see Example 10-4). 

Example 10-4. WSDL definition of the portType for the ping operation 

<portType name="pingPort"> 
<operation name="ping"> 
<input message="tns : pingRequest"/> 
<output message="tns : pingResponse"/> 
</operation> 
</portType> 

At this point, the definition is still abstract. There are any number of ways to implement 
this ping operation that takes two strings and returns a struct. I haven't specified that 
the message is going to be transported via SOAP, or even that the message is going to 
be XML. Vendors of WSDL implementations are free to support other transports, but 
in Example 10-5, the intended binding is to section 5 compliant SOAP messages, send 
over HTTP. This is the SOAP/HTTP binding for the port type, which will be presented 
without commentary. 

Example 1 0-5. Binding the ping portType to a SOAP/HTTP implementation 

<binding name="pingSoap" type="tns:pingPort"> 
<soap:binding style="rpc" transport="http://schemas. xmlsoap.org/soap/http" /> 
<operation name="ping"> 

<soap:operation soapAction="/weblogllpdates" style="rpc"/> 

<input> 

<soap:body use="encoded" namespace="uri:weblogscom" 
encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/> 
</input> 
<output> 

<soap:body use="encoded" namespace="uri:weblogscom" 
encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/> 
</output> 
</operation> 
</binding> 

We're still not done. The final piece to the puzzle is to define a service, which connects 
a portType with a binding and (since this is SOAP over HTTP) with an endpoint URI 
(see Example 10-6). 

Example 10-6. Defining a SOAP service that exposes the ping port 

<service name="weblogscom"> 
<document> 

For a complete description of this service, go to the following URL: 
http://www.soapware.org/weblogsCom 
</document> 

<port name="pingPort" binding="tns :pingSoap"> 

<soap : address location="http : //rpc.weblogs . com:8o/"/> 
</port> 
</service> 



WSDL | 305 



The full WSDL for this single-function service is shown in Example 10-7. 

Example 10-7. The complete WSDL file 

<?xml version="l.O" encoding="utf-8"?> 

definitions 

xmlns : soap= " http : / /schemas . xmlsoap . org/wsdl/ soap/" 

xmlns: s=" http://www.w3.org/200l/XMLSchema" 

xmlns :tns="uri:weblogscom" 

targetNamespace="uri:weblogscom" 

xmlns= " http : //schemas . xmlsoap . org/wsdl/ " > 

<types> 

<s: schema targetNamespace="uri:weblogscom"> 
<s:complexType name="pingResult"> 
<s:sequence> 
<s:element minOccurs="l" maxOccurs="l" 

name= "f lerror " type= " s : boolean "/> 
<s:element minOccurs="l" maxOccurs="l" 
name="message" type="s: string" /> 
</s:sequence> 

</s:complexType> 
</s:schema> 
</types> 

<message name="pingRequest"> 

<part name="weblogname" type="s:string"/> 

<part name="weblogurl" type="s:string"/> 
</message> 

<message name="pingResponse"> 

<part name="result" type="tns :pingResult"/> 
</message> 

<portType name="pingPort"> 
<operation name="ping"> 
<input message="tns:pingRequest"/> 
<output message="tns : pingResponse"/> 
</operation> 
</portType> 

<binding name="pingSoap" type="tns :pingPort"> 
<soap: binding style="rpc" 

transport="http: //schemas. xmlsoap. org/soap/http"/> 
<operation name="ping"> 

<soap:operation soapAction="/weblogUpdates" style="rpc"/> 

<input> 

<soap:body use="encoded" namespace="uri:weblogscom" 
encodingStyle="http: //schemas. xmlsoap. org/soap/encoding/"/> 
</input> 
<output> 

<soap:body use="encoded" namespace="uri:weblogscom" 
encodingStyle="http: //schemas. xmlsoap. org/soap/encoding/"/> 
</output> 

</operation> 



306 I Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



</binding> 



<service name="weblogscom"> 
<document> 

For a complete description of this service, go to the following 
URL : http : / /www. soapware . org/weblogsCom 
</document> 

<port name="pingPort" binding="tns:pingSoap"> 

<soap: address location="http://rpc.weblogs.com:8o/"/> 
</port> 
</service> 

</definitions> 

Frankly, that's a lot of work for a single operation that accepts two string parameters 
and returns a Boolean and a string. I had to do all this work because WSDL makes no 
simplifying assumptions. I started off specifying the request and response in the ab- 
stract. Then I had to bind them together into an operation. I exposed the operation as 
a portType, I defined a port of that type that accepted SOAP messages through HTTP, 
and then I had to expose that port at a specific URI. For this simple case, creating the 
WSDL by hand is possible (I just did it) but difficult. That's why most WSDL is gen- 
erated by automated tools. For simple services you can start from a generated WSDL 
file and tweak it slightly, but beyond that you're at the mercy of your tools. 

The tools then become the real story. They abstract away the service, the binding, the 
portType, the messages, the schema, and even the network itself. If you are coding in a 
statically typed language, like C# or Java, you can have all this WSDL generated for 
you at the push of a button. Generally all you have to do is select which methods in 
which classes you want exposed as a web service. Almost all WSDL today is generated 
by tools and can only be understood by tools. After some setup, the client's tools can 
call your methods through a web service and it looks like they're calling native-language 
methods. 

What's not to like? How is this different from a compiler, which turns high-level con- 
cepts into machine code? 

What ought to concern you is that you're moving further and further away from the 
Web. Machine code is no substitute for a high-level language, but the Web is already 
a perfectly good platform for distributed programming. That's the whole point of this 
book. This way of exposing programming-language methods as web services encour- 
ages an RPC style that has the overhead of HTTP, but doesn't get any of the benefits 
Fve been talking about. 

Even new WSDL features like document/literal encoding (which I haven't covered here) 
encourage the RPC style of web services: one where every method is a POST, and one 
where URIs are overloaded to support multiple operations. It's theoretically possible 
to define a fully RESTful and resource-oriented web service in WSDL (something that's 
even more possible with WSDL 2.0). It's also theoretically possible to stand an egg on 



WSDL | 307 



end on a flat surface. You can do it, but you'll be fighting your environment the whole 
way. 

Generated SOAP/WSDL interfaces also tend to be brittle. Different Big Web Services 
stacks interpret the standards differently, generate slightly different WSDL files, and 
can't understand each other's messages. The result is that clients are tightly bound to 
servers that use the same stack. Web services ought to be loosely coupled and resilient: 
they're being exposed across a network to clients who might be using a totally different 
set of software tools. The web has already proven its ability to meet this goal. 

Worst of all, none of the complexities of WSDL help address the travel broker scenario. 
Solving the travel broker problem requires solving a number of business problems, like 
getting "a five-minute hold on seat 24C." Strong typing and protocol independence 
aren't the solution to any of these problems. Sometimes these requirements can be 
justified on their own terms, but a lot of the time they go unnoticed and unchallenged, 
silently dragging on other requirements like simplicity and scalability. 

The Resource-Oriented Alternative 

WSDL serves two main purposes in real web services. It describes which interface 
(which RPC-style functions) the service exposes. It also describes the representation 
formats: the schemas for the XML documents the service accepts and sends. In re- 
source-oriented services, these functions are often unnecessary or can be handled with 
much simpler standards. 

From a RESTful perspective, the biggest problem with WSDL is what kind of interface 
it's good at describing. WSDL encourages service designers to group many custom 
operations into a single "endpoint" that doesn't respond to any uniform interface. Since 
all this functionality is accessible through overloaded POST on a single endpoint URI, 
the resulting service isn't addressable. WADL is an alternative service description lan- 
guage that's more in line with the Web. Rather than describing RPC-style function calls, 
it describes resources that respond to HTTP's uniform interface. 

WSDL also has no provisions for defining hypertext links, beyond the anylJRI data 
type built into XML Schema. SOAP services aren't well connected. How could they be, 
when an entire service is hidden behind a single address? Again, WADL solves this 
problem, describing how one resource links to another. 

A lot of the time you don't need to describe your representation formats at all. In many 
Ajax applications, the client and server ends are written by the same group of people. 
If all you're doing is serializing a data structure for transport across the wire (as happens 
in the weblogs.com ping service), consider JSON as your representation format. You 
can represent fairly complex data structures in JSON without defining a schema; you 
don't even need to use XML. 

Even when you do need XML, you often find yourself not needing a formally defined 
schema. Sprinkled throughout this book are numerous examples of clients that use 



308 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



XPath expressions like "/posts/post" to extract a desired chunk out of a larger XML 
document. These short strings are often the only description of an XML document a 
client needs. 

There's nothing unRESTful or un-resource-oriented about XML Schema definitions. 
A schema definition is often overkill, but if it's the right tool for the job, use it. I just 
think it shouldn't be required. 

UDDI 

A full description of UDDI is way beyond the scope of this book. Think of it as a yellow 
pages for WSDL, a way for clients to look up a service that fits their needs. UDDI is 
even more complex than WSDL. The UDDI specification defines a four-tier hierarchical 
XML schema that provides metadata about web service descriptions. The data structure 
types you'll find in a UDDI registry are a businessEntity , a businessService, a binding- 
Template, and a tModel. 

The vision of UDDI was one of multiple registries: a fully-replicated Internet-scale reg- 
istry for businesses, and a private registry behind the firewall of every company that 
wanted to host one. In 2006, IBM and Microsoft shut down their public UDDI registry 
after publicly declaring it a success. The IBM/Microsoft registry was reported to de- 
scribe 50,000 businesses, but privately it was recognized that not all of that data was 
properly vetted, which inhibited adoption. 

So sheer complexity is not the only reason why public adoption of UDDI never caught 
on. This is just speculation, but additional factors were probably the relatively small 
number of public SOAP services, successful companies' general desire to not commo- 
ditize themselves, and WSDL's tendency to promote a unique interface for every web 
service. Which is a shame, as UDDI could definitely have helped travel brokers find 
independently operated hotels. UDDI has seen greater success within companies, 
where it's practical to impose quality controls and impose uniform interfaces. 

The Resource-Oriented Alternative 

There's no magic bullet here. Any automated system that helps people find hotels has 
a built-in economic incentive for hotel chains to game the system. This doesn't mean 
that computers can't assist in the process, but it does mean that a human needs to make 
the ultimate decision. 

The closest RESTful equivalents to UDDI are the search engines, like Google, Yahoo!, 
and MSN. These help (human) clients find the resources they're looking for. They take 
advantage of the uniform interface and common data formats promoted by REST. Even 
this isn't perfect: spammers try to game the search engines, and sometimes they suc- 
ceed. But think of the value of search engines and you'll see the promise of UDDI, even 
if its complexity turns you off. 



UDDI | 309 



As RESTful web services grow in popularity and become better-connected (both in- 
ternally and to the Web at large), something like today's search engines may fulfill the 
promise of the public UDDI registry. Instead of searching for services that expose cer- 
tain APIs, we'll search for resources that accept or serve representations with certain 
semantics. Again, this is speculation. Right now, the public directories of web services 
(I list a few in Appendix A) are oriented toward use by human beings. 

Security 

"Security" evokes a lot of related concepts: signatures, encryption, keys, trust, federa- 
tion, and identity. HTTP's security techniques focus pretty much exclusively on au- 
thentication and the transfer of messages. The collection of WS-* specifications related 
to security (and they are numerous) attempt to cover a more complete picture. 

The simplest application of WS-Security is the UserName token profile. This is a SOAP 
"sticker" that goes on the envelope to give some context to the request: in this case, the 
sticker explains who's making the request (see Example 10-8). 

Example 10-8. The UserName token: a SOAP sticker 

<Security wsse=" http://schemas.xmlsoap.org/ws/2002/xx/secext"> 
<UsernameToken> 

<Username>Zoe</Username> 
<Password>ILoveDogs</Password> 
</UsernameToken> 
</Security> 

When placed inside of the Header section of a SOAP message, this conveys a set of 
authentication credentials. It has the same qualities as HTTP Basic authentication, an 
HTTP sticker which goes into the Authorization HTTP header. 

Passing passwords in clear text is not exactly best practice, especially if the channel 
isn't secure. WS-Security defines a number of alternatives. To oversimplify considera- 
bly, the WS-Security specification defines a consistent set of XML element names for 
conveying concepts defined in other standards: passwords, SAML tokens, X.509 to- 
kens, Kerberos tokens, and the like. There's no reason that a similar effort couldn't be 
undertaken to map similar concepts to HTTP headers. HTTP authentication is exten- 
sible, and in the early days of the development of Atom, some WS-Security concepts 
were ported to HTTP as WSSE (again, see "Authentication and Authorization" in 
Chapter 8) . 

But Big Web Services security involves more than the WS-Security standard. Two 
examples: 

• Signatures can enable nonrepudiation. It's possible to prove the originator of a 
given message was long after it sent, and that the message was not modified after 
it was received. These concepts are important in contracts and checks. 



310 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



• Federation enables a third party to broker trust of identities. This would allow a 
travel broker to verify that a given person works for one of the travel broker's 
customers: this might affect billing and discounts. 

More examples are well beyond the scope of this book. Suffice it to say that security 
concepts are much better specified and deployed in SOAP -based protocols than in 
native HTTP protocols. That doesn't mean that this gap can't be closed, that SOAP 
stickers can't be ported to HTTP stickers, or that one-off solutions are possible without 
SOAP. Right now, though, SOAP has many security-related stickers that HTTP doesn't 
have, and these stickers are useful when implementing applications like the travel 
broker. 

As a caution, many of these areas are not areas where amateurs can productively dabble. 
Nobody should try to add new security concepts to HTTP all by themselves. 

The Resource-Oriented Alternative 

An application is only as secure as its weakest link. If you encrypt a credit card number 
for transport over the wire and then simply store it in a database, all you've done is 
ensure that attackers will target your database. Your view of security needs to encom- 
pass the entire system, not just the bits transmitted over the network. 

That said, the WS-Security family of specifications are not the only tools for securing 
those bits. HTTPS (a.k.a Transport Layer Security [TLS], a.k.a. Secure Sockets Layer 
[SSL]) has proven sufficient in practice for securing credit card information as it's sent 
across the network. People trust their credit cards to SSL all the time, and the vast 
majority of attacks don't involve breaking SSL. The use of XML signatures and en- 
cryption is also not limited to WS-*. Section 5 of the Atom Syndication Format standard 
shows how to use these features in Atom documents. You've also seen how S3 imple- 
ments request signing and access control in Chapter 3. These aspects of security are 
possible, and have been deployed, in RESTful resource-oriented services. But no one's 
done the work to make these features available in general. 

When all is said and done, your best protection may be the fact that resource-oriented 
architectures promote simplicity and uniformity. When you're trying to build a secure 
application, neither complexity nor a large number of interfaces turn out to be advan- 
tages. 

Reliable Messaging 

The WS-ReliableMessaging standard tries to provide assurances to an application that 
a sequence of messages will be delivered AtMostOnce, AtLeastOnce, ExactlyOnce, or 
InOrder. It defines some new headers (that is, stickers on the envelope) that track 
sequence identifiers and message numbers, and some retry logic. 



Reliable Messaging | 311 



The Resource-Oriented Alternative 

Again, these are areas where the specification and implementation for SOAP -based 
protocols are further advanced than those for native HTTP. In this case, there is an 
important difference. When used in a certain way, HTTP doesn't need these stickers 
at all. 

As I said earlier, almost all of the HTTP methods are idempotent. If a GET, HEAD, 
PUT, or DELETE operation doesn't go through, or you don't know whether or not it 
went through, the appropriate course of action is to just retry the request. With idem- 
potent operations, there's no difference between AtMostOnce, AtLeastOnce, and Ex- 
actlyOnce. To get InOrder you just send the messages in order, making sure that each 
one goes through. 

The only nonidempotent method is POST, the one that SOAP uses. SOAP solves the 
reliable delivery problem from scratch, by defining extra stickers. In a RESTful appli- 
cation, if you want reliable messaging for all operations, I recommend implementing 
POST Once Exactly (covered back in Chapter 9) or getting rid of POST altogether. The 
WS-ReliableMessaging standard is motivated mainly by complex scenarios that REST- 
ful web services don't address at all. These might be situations where a message is 
routed through multiple protocols on the way to its destination, or where both source 
and destination are cell phones with intermittent access to the network. 

Transactions 

Transactions are simple to describe, but insanely difficult to implement, particularly in 
a distributed environment. The idea is that you have a set of operations: say, "transfer 
$50 from bank A to bank B," and the entire operation must either succeed or fail. Bank 
A and bank B compete with each other and expose separate web services. You either 
want bank A to be debited and bank B to be credited, or you want nothing to happen 
at all. Neither debiting without crediting, or crediting without debiting are desirable 
outcomes. 

There are two basic approaches. The WS-AtomicTransaction standard specifies a com- 
mon algorithm called a two-phase commit. In general, this is only wise between parties 
that trust one another, but it's the easiest to implement, it falls within the scope of 
existing products, and therefore it's the one that is most widely deployed. 

The second approach is defined by WS-BusinessActivity, and it more closely follows 
how businesses actually work. If you deposit a check from a foreign bank, your bank 
may put a hold on it and seek confirmation from the foreign bank. If it hears about a 
problem before the hold expires, it rolls back the transaction. Otherwise, it accepts the 
check. If it happens to hear about a problem after it's committed the transaction, it 
creates a compensating transaction to undo the deposit. The focus is on undoing mis- 
takes in an auditable way, not just preventing them from happening. 



312 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



The Resource-Oriented Alternative 

Again, there's not much that corresponds to this level of specification and deployment 
in native HTTP applications. It's usually not necessary at all. In Chapter 8 I imple- 
mented a transaction system by exposing the transactions as resources, but I didn't 
need two-phase commit because there was only one party to the transaction. I was 
transferring money between accounts in a single bank. But if a number of web services 
supported this kind of transaction, I could stick a little bit of infrastructure on top and 
then orchestrate them with RESTful two-phase commit. 

Two-phase commit requires a level of control over and trust in the services you're 
coordinating. This works well when all the services are yours, but not so well when you 
need to work with a competing bank. SOA architects think two-phase commit is in- 
appropriate for web service-based interactions in general, and I think it's usually inap- 
propriate for RESTful web services. When you don't control the services you're coor- 
dinating, I recommend implementing the ideas behind WS-BusinessActivity with asyn- 
chronous operations (again, from Chapter 8). 

To go back to the example of the check from a foreign bank: your bank might create a 
"job" resource on the foreign bank's web service, asking if the check is valid. After a 
week with no updates to that resource, your bank might provisionally accept the check. 
If two days later the foreign bank updates the "job" resource saying that the check is 
bad, your bank can create a compensating transaction, possibly triggering an overdraft 
and other alerts. You probably won't need to create a complex scenario like this, but 
you can see how patterns I've already demonstrated can be used to implement these 
new ideas. 

BPEL, ESB, and SOA 

Implemented on top of the foundation I just described are some concepts that are 
controversial even in the world of Big Web Services. I'll cover them briefly here. 

Business Process Execution Language (BPEL) is an XML grammar that makes it pos- 
sible to describe business processes that span multiple parties. The processes can be 
orchestrated automatically via software and web services. 

The definition of an Enterprise Service Bus (ESB) varies, but tends to include discovery, 
load balancing, routing, bridging, transformation, and management of web service re- 
quests. This often leads to a separation of operations from development, making each 
simpler and easier to run. 

The downside of BPEL and ESB is that they tend to increase coupling with, and reliance 
on, common third-party middleware. One upside is that you have a number of choices 
in middleware, varying from well-supported open source offerings to ones provided by 
established and recognized proprietary vendors. 



BPEL, ESB, and SOA | 313 



Service-Oriented Architecture (SOA) is perhaps the least well-defined term of all, which 
is why I called it out in Chapter 1 as a term I wasn't going to use. I know of no litmus 
test which indicates whether a given implementation is SOA or not. Sometimes a dis- 
cussion of SOA starts off saying that SOA encompasses all REST/HTTP applications, 
but inevitably the focus turns to the Big Web Services standards I described in this 
chapter. 

That said, one aspect of SOA is noteworthy. To date, many approaches to distributed 
programming focus on remote procedure calls, striving to make them as indistinguish- 
able from local procedure calls as humanly possible. An example is a WSDL file gen- 
erated from a preexisting application. The SOA idea at least returns the focus to inter- 
faces: in particular, to interfaces that span machine boundaries. Machine boundaries 
tend to not happen by accident. They often correlate to trust boundaries, and they're 
the places where message reliability tends to be an issue. Machine boundaries should 
be studied, not abstracted away 

Some other aspects of SOA are independent of the technical architecture of a service. 
They can be implemented in resource-oriented environments, environments full of Re- 
mote Procedure Call services, or heterogeneous environments. "Governance," for ex- 
ample, has to do with auditing and conformance to policies. These "policies" can be 
anything from government regulations to architectural principles. One possible policy 
might be: "Don't make RPC-style web service calls." 

Conclusion 

Both REST and web services have become buzzwords. They are chic and fashionable. 
These terms are artfully woven into PowerPoint presentations by people who have no 
real understanding of the subject. This chapter, and indeed this book, is an attempt to 
dispel some of the confusion. 

In this chapter you've seen firsthand the value that SOAP brings (not so much), and 
the complexity that WSDL brings (way too much). You've also seen resource-oriented 
alternatives listed every step of the way. Hopefully this will help you make better 
choices. If you can see you'll need some of the features described in this chapter which 
are only available as stickers on SOAP envelopes, getting started on the SOAP path 
from the beginning will provide a basis for you to build on. 

The alternative is to start lightweight and apply the YAGNI (You Aren't Gonna Need 
It) principle, adding only the features that you know you actually need. If it turns out 
you need some of the stickers that only Big Web Services can provide, you can always 
wrap your XML representations in SOAP envelopes, or cherry-pick the stickers you 
need and port them to HTTP headers. Given the proven scalability of the Web, starting 
simple is usually a safe choice — safe enough, I think, to be the default. 



314 | Chapter 10: The Resource-Oriented Architecture Versus Big Web Services 



CHAPTER 11 



Ajax Applications as REST Clients 



Ajax applications have become very hot during the past couple of years. Significantly 
hotter, in fact, than even knowing what Ajax applications are. Fortunately, once you 
understand the themes of this book it's easy to explain Ajax in those terms. At the risk 
of seeming old-fashioned, I'd like to present a formal definition of Ajax: 

An Ajax application is a web service client that runs inside a web browser. 

Does this make sense? Consider two examples widely accepted not to be Ajax appli- 
cations: a JavaScript form validator and a Flash graphics demo. Both run inside the web 
browser, but they don't make programmatic HTTP requests, so they're not Ajax ap- 
plications. On the flip side: the standalone clients I wrote in Chapters2 and 3 aren't 
Ajax applications because they don't run inside a web browser. 

Now consider Gmail, a site that everyone agrees uses Ajax. If you log into Gmail you 
can watch your browser make background requests to the web service at mail. goo 
gle.com, and update the web page you're seeing with new data. That's exactly what a 
web service client does. The Gmail web service has no public-facing name and is not 
intended for use by clients other than the Gmail web page, but it's a web service none- 
theless. Don't believe it? There are libraries like libgmail (http: //lib gmail. source 
forge.net/) that act as unofficial, non-Ajax clients to the Gmail web service. Remember, 
if it's on the Web, it's a web service. 

This chapter covers client programming, and it picks up where Chapter 2 left off. Here 
I'm focusing on the special powers and needs of web service clients that run in a browser 
environment. I cover JavaScript's XMLHttpRequest class and the browser's DOM, and 
show how security settings affect which web service clients you can run in a browser. 

From AJAX to Ajax 

Every introduction to Ajax will tell you that it used to be AJAX, an acronym for Asyn- 
chronous JavaScript And XML. The acronym has been decommissioned and now Ajax 
is just a word. It's worth spending a little time exploring why this happened. Program- 
mers didn't suddenly lose interest in acronyms. AJAX had to be abandoned because 



315 



what it says isn't necessarily true. Ajax is an architectural style that doesn't need to 
involve JavaScript or XML. 

The JavaScript in AJAX actually means whatever browser-side language is making the 
HTTP requests. This is usually JavaScript, but it can be any language the browser knows 
how to interpret. Other possibilities are ActionScript (running within a Flash applica- 
tion), Java (running within an applet), and browser-specific languages like Internet 
Explorer's VBScript. 

XML actually means whatever representation format the web service is sending. This 
can be any format, so long as the browser side can understand it. Again, this is usual- 
ly XML, because it's easy for browsers to parse, and because web services tend to serve 
XML representations. But JSON is also very common, and it can be also be HTML, 
plain text, or image files: anything the browser can handle or the browser-side script 
can parse. 

So AJAX hackers decided to become Ajax hackers, rather than always having to explain 
that JavaScript needn't mean JavaScript and XML might not be XML, or becoming 
Client-Side Scripting And Representation Format hackers. When I talk about Ajax in 
this book I mostly talk in terms of JavaScript and XML, but Fm not talking about those 
technologies: Fm talking about an application architecture. 

The Ajax Architecture 

The Ajax architecture works something like this: 

1. A user, controlling a browser, makes a request for the main URI of an application. 

2. The server serves a web page that contains an embedded script. 

3. The browser renders the web page and either runs the script, or waits for the user 
to trigger one of the script's actions with a keyboard or mouse operation. 

4. The script makes an asynchronous HTTP request to some URI on the server. The 
user can do other things while the request is being made, and is probably not even 
aware that the request is happening. 

5. The script parses the HTTP response and uses the data to modify the user's view. 
This might mean using DOM methods to change the tag structure of the original 
HTML page. It might mean modifying what's displayed inside a Flash application 
or Java applet. 

From the user's point of view, it looks like the GUI just modified itself. 

This architecture looks a lot like that of a client-side GUI application. In fact, that's 
what this is. The web browser provides the GUI elements (as described in your initial 
HTML file) and the event loop (through JavaScript events). The user triggers events, 
which get data from elsewhere and alter the GUI elements to fit. This is why Ajax 
applications are often praised as working like desktop applications: they have the same 
architecture. 



316 | Chapter 11: Ajax Applications as REST Clients 



A standard web application has the same GUI elements but a simpler event loop. Every 
click or form submission causes a refresh of the entire view. The browser gets a new 
HTML page and constructs a whole new set of GUI elements. In an Ajax application, 
the GUI can change a little bit at a time. This saves bandwidth and reduces the psy- 
chological effects on the end user. The application appears to change incrementally 
instead of in sudden jerks. 

The downside is that every application state has the same URI: the first one the end 
user visited. Addressability and statelessness are destroyed. The underlying web service 
may be addressable and stateless, but the end user can no longer bookmark a particular 
state, and the browser's "Back" button stops working the way it should. The application 
is no longer on the Web, any more than a SOAP+WSDL web service that only exposes 
a single URI is on the Web. I discuss what to do about this next. 

Adel.icio.us Example 

Back in Chapter 2 1 showed clients in various languages for a REST-RPC hybrid service: 
the API for the del.icio.us social bookmarking application. Though I implemented my 
own, fully RESTful version of that service in Chapter 7, I'm going to bring the original 
service out one more time to demonstrate a client written in JavaScript. Like most 
JavaScript programs, this one runs in a web browser, and since it's a web service client, 
that makes it an Ajax application. Although simple, this program brings up almost all 
of the advantages of and problems with Ajax that I discuss in this chapter. 

The first part of the application is the user interface, implemented in plain HTML. This 
is quite different from my other del.icio.us clients, which ran on the command line and 
wrote their data to standard output (see Example 11-1). 

Example 11-1. An Ajax client to the del.icio.us web service 
<!D0CTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 

Transitional//EN" "http://www.w3.org/TR/REC-html40/transitional.dtd"> 
< ! --delicious -ajax. html-- > 

<!--An Ajax application that uses the del.icio.us web service. This 
application will probably only work when saved as a local 
file. Even then, your browser's security policy might prevent it 
from running. --> 

<html> 
<head> 

<title>DavaScript del. icio. us</title> 
</head> 
<body> 

<hl>3avaScript del.icio.us example</hl> 

<p>Enter your del.icio.us account information, and I'll fetch and 
display your most recent bookmarks. </p> 

<form onsubmit="callDelicious() ; return false; "> 
Username: <input id="username" type="text" /xbr /> 



A del.icio.us Example | 317 



Password: <input id="password" type="password" /xbr /> 
<input type="submit" value="Fetch del.icio.us bookmarks "/> 
</-form> 

<div id="message"x/div> 
<ul id="links"x/ul> 

My user interface is an HTML form that doesn't point anywhere, and some tags (div 
and ul) that don't contain anything. I'm going to manipulate these tags with JavaScript 
functions. The first is setHessage, which puts a given string into the div tag (see Ex- 
ample 11-2). 

Example 11-2. Ajax client continued: definition of setMessage 

<script type= "text/ javascript "> 
function setMessage(newValue) { 
message = document. getElementById("message"); 
message. firstChild.textContent = newValue; 

} 

And it's not quite fair to say that the HTML form doesn't point anywhere. Sure, it 
doesn't have an "action" attribute like a normal HTML form, but it does have an 
onsubmit event handler. This means the web browser will call the JavaScript function 
callDelicious whenever the end user clicks the submit button. Instead of going through 
the page request loop of a web browser, I'm using the GUI-like event loop of a JavaScript 
program. 

The callDelicious function uses the JavaScript library XMLHttpRequest to fetch data 
from https://api.del.icio.us/vl/posts/recent/. This is the URI used throughout Chap- 
ter 2 to fetch a user's most recent del.icio.us bookmarks. First we need to do some 
housekeeping: get permission from the browser to send the request, and gather what- 
ever data the user entered into the HTML form (see Example 11-3). 

Example 11-3. Ajax client continued: definition of callDelicious 

function callDelicious() { // Get permission from the browser to send the request, 
try { 

if (netscape. security. PrivilegeManager.enablePrivilege) 
netscape. security. PrivilegeManager.enablePrivilege("UniversalBrowserRead"); 
} catch (e) { 

alert("Sorry, browser security settings won't let this program run."); 
return; 

} 

// Fetch the user-entered account information 

var username = document. getElementById("username") .value; 

var password = document. getElementById("password") .value; 

// Remove any old links from the list. 

var links = document. getElementById("links"); 

while (links. firstChild) 



318 | Chapter 11: Ajax Applications as REST Clients 



links. removeChild( links. firstChild) 
setMessage("Please wait..."); 

Now we're ready to send the HTTP request, as shown in Example 11-4. 

Example 11-4. callDelicious definition continued 
II Send the request. 

// See "Working Around the Corner Cases" for a cross-browser 
// "createXMLHttpRequest" implementation, 
request = new XMLHttpRequestQ; 

request. open ("GET", "https://api.del.icio.us/vl/posts/recent", true, 

username, password); 
request. onreadystatechange = populateLinkList; 
request. send(null); 

The third JavaScript function I'll define is populateLinkList. I've already referenced 
this function, in the line request. onreadystatechange = populateLinkList. That line 
sets up populateLinkList as a callback function. The idea is that while 
api.del.icio.us is processing the request, the user can go about her business, surfing 
the web in another browser window. Once the request completes, the browser calls 
populateLinkList, which handles the response. You can do JavaScript programming 
without these callback functions, but it's a bad idea. Without callbacks, the web 
browser will go nonresponsive while the XMLHttpRequest object is making an HTTP 
request. Not very asynchronous. 

The job of populateLinkList is to parse the XML document from the del.icio.us web 
service. The representation in Example 11-5 represents a list of bookmarks, and 
populateLinkList turns each bookmark into a list item of the formerly empty ul list tag. 

Example 11-5. Ajax client concluded: definition of populateLinkList 

II Called when the HTTP request has completed, 
function populateLinkListQ { 
if (request. readyState != 4) // Request has not yet completed 
return; 

setMessage("Request complete. "); 

if (net scape. security. PrivilegeManager.enablePrivilege) 

netscape . security . PrivilegeManager . enablePrivilege ( " UniversalBrowserRead " ) ; 
// Find the "post" tags in the representation 
posts = request. responseXML.getElementsByTagName("post"); 

setMessage(posts. length + " link(s) found:"); 
// For every "post" tag in the XML document... 
for (var i = 0; i < posts. length; i++) { 
post = posts[i]; 

// ...create a link that links to the appropriate URI. 
var link = document. createElement("a"); 
var description = post. getAttribute( 'description' ); 
link. set Attribute ("href", post.getAttribute( ' href )); 
link. appendChild (document. createTextNode(description)); 

// Stick the link in an "li" tag... 



A del.icio.us Example | 319 



var listltem = document. createElement("li"); 

// ...and make the "li" tag a child of the "ul" tag. 

listltem. appendChild (link); links. appendChild( list Item) 

} 

} 

} 

</script> 
</body> 
</html> 

The Advantages of Ajax 

If you try out the del.icio.us client you'll notice some nice features that come from the 
web browser environment. Most obviously: unlike the examples in Chapter 2, this 
application has a GUI. And as GUI programming goes, this is pretty easy. Method calls 
that seem to do nothing but manipulate a mysterious document data structure, actually 
change the end user's view of the application. The document is just the thing the user 
sees rendered in the browser. Since the browser knows how to turn changes to the 
document into GUI layout changes, there's no widget creation and layout specification, 
as you'd see in conventional GUI programs. 

This client also never explicitly parses the XML response from the del.icio.us web serv- 
ice. A web browser has an XML parser built in, and XMLHttpRequest automatically parses 
into a DOM object any XML document that comes in on a web service response. You 
access the DOM object through the XMLHttpRequest. responseXML member. The DOM 
standard for web browsers defines the API for this object: you can iterate over its chil- 
dren, search it with methods like getElementsByTagName, or hit it with XPath 
expressions. 

More subtly: try loading this HTML file and clicking the submit button without pro- 
viding a username and password. You'll get a dialog box asking you for a del.icio.us 
username and password: the same dialog box you get whenever your browser accesses 
a page that requires HTTP basic auth. This is exactly what you're doing: visiting https:// 
api.del.icio.us/vl/posts/recent, which requires HTTP basic auth, in your web browser. 
But now you're doing it by triggering an action in an Ajax application, rather than 
clicking on a link to the URL 

Web browsers are by far the most popular HTTP clients out there, and they've been 
written to handle the corner cases of HTTP. You could remove both text fields from 
the HTML form in Example 11-1, and the Ajax application would still work, because 
real web browsers have their own user interface for gathering basic HTTP auth 
information. 



320 | Chapter 11: Ajax Applications as REST Clients 



The Disadvantages of Ajax 

Unfortunately, thanks to the wide variety of web browsers in use, you'll need to deal 
with a whole new set of corner cases if you want your application to work in all brows- 
ers. Later on I'll show you code libraries and code snippets that work around the corner 
cases. 

If you try out this program, you'll also run into the problem I talked about at the end 
of Chapter 8: why should the end user trust the web service client? You'd trust your 
browser with your del.icio.us username and password, but this isn't your browser. It's 
a web service client that uses your browser to make HTTP requests, and it could be 
doing anything in those requests. If this was an official web page that was itself served 
from api. del.icio.us, then your browser would trust it to make web service calls to 
the server it came from. But it's a web page that comes from a file on your hard drive, 
and wants to call out to the Web at large. To a web browser, this is very suspicious 
behavior. 

From a security standpoint, this is no different from the standalone del.icio.us clients 
I wrote in other programming languages. But there's no real reason why you should 
trust a standalone web service client, either. We just tend to assume they're safe. A web 
browser is constantly loading untrusted web pages, so it has a security model that 
restricts what those pages can do in JavaScript. If strangers were always dumping ex- 
ecutables into your home directory, you'd probably think twice before running them. 

Which is why I called netscape. security. PrivilegeManager .enablePrivilege, asking 
the browser if it won't let me make an HTTP request to a foreign domain ("Universal- 
BrowserRead"), and won't it also let me use the browser's XML parser on some data 
from a foreign domain ("UniversalBrowserRead" again, but in a different JavaScript 
function). Even with these calls in place, you're likely to get browser security messages 
asking you if you want to accept this risky behavior. (These are not like the browser 
messages you might get when you do something innocuous like submit an HTML form, 
messages that Justin Mason once characterized as "are you sure you want to send stuff 
on the intarweb?". These are more serious.) And that's with this file sitting on your 
(presumably trusted) filesystem. If I tried to serve this Ajax application from 
oreilly.com, there's no way your browser would let it make an HTTP request to 
api. del.icio.us. 

So why don't we see these problems all the time in Ajax applications? Because right 
now, most Ajax applications are served from the same domain names as the web serv- 
ices they access. This is the fundamental difference between JavaScript web service 
clients and clients written in other languages: the client and the server are usually writ- 
ten by the same people and served from the same domain. 

The browser's security model doesn't totally prevent you from writing an XMLHttpRe 
quest application against someone else's web service, but it does make it difficult. Ac- 
cording to the web browser, the only Ajax application safe enough to run without a 



The Disadvantages of Ajax | 321 



warning is one that only makes requests against the domain it was served from. At the 
end of this chapter I'll show you ways of writing Ajax clients that can consume foreign 
web services. Note, though, that these techniques rely heavily on cheating. 

REST Goes Better 

Ajax applications are web service clients, but why should they be clients of RESTful 
web services in particular? Most Ajax applications consume a web service written by 
the same people who wrote the application, mainly because the browser security model 
makes it difficult to do anything else. Why should it matter whether a service used by 
one client is fully RESTful or just a resource-oriented/RPC hybrid? There are even pro- 
grams that turn a WSDL file into a JavaScript library for making RPC SOAP calls 
through XMLHttpRequest. What's wrong with that? 

Well, in general, the interface between two parts of an application matters. If RESTful 
architectures yield better web services, then you'll benefit from using them, even if 
you're the only one consuming the service. What's more, if your application does 
something useful, people will figure out your web service and write their own clients 
— just as if your web site exposes useful information, people will screen-scrape it. Un- 
less you want to obfuscate your web service so only you can use it, I think the Resource- 
Oriented Architecture is the best design. 

The web services that Ajax applications consume should be RESTful for the same rea- 
sons almost all web services should be RESTful: addressability, statelessness, and the 
rest. The only twist here is that Ajax clients are embedded inside a web browser. And 
in general, the web browser environment strengthens the argument for REST. You 
probably don't need me to reiterate my opinion of Big Web Services in this chapter, 
but SOAP, WSDL, and the rest of the gang look even more unwieldy inside a web 
browser. Maybe you're a skeptic and you think the REST paradigm isn't suitable as a 
general platform for distributed programming — but it should at least be suitable for 
the communication between a web browser and a web server! 

Outside of a web browser, you might decide to limit yourself to the human web's 
interface of GET and POST. Many client libraries support only the basic features of 
HTTP. But every Ajax application runs inside a capable HTTP client. The chart below 
has the details, but almost every web browser gives XMLHttpRequest access to the five 
basic HTTP methods, and they all let you customize the request headers and body. 

What's more, Ajax calls take place in the same environment as the end user's other web 
browsing. If the client needs to make HTTP requests through a proxy, you can assume 
they've already configured it. An Ajax request sends the same cookies and Basic auth 
headers as do other browser requests to your domain. You can usually use the same 
authentication mechanisms and user accounts for your web site and your Ajax services. 

Look back at steps 4 and 5 of the Ajax architecture — basically "GET a URI" and "use 
data from the URI to modify the view." That fits in quite well with the Resource- 



322 | Chapter 11: Ajax Applications as REST Clients 



Oriented Architecture. An Ajax application can aggregate information about a large 
number of resources, and incrementally change the GUI as the resource state changes. 
The architectural advantages of REST apply to Ajax clients just as they do to other 
clients. One example: you don't need to coordinate the browser's application state with 
the server if the server never keeps any application state. 

Making the Request 

Now I'd like to look at the technical details underlying the most common client lan- 
guage for Ajax: JavaScript. The major web browsers all implement a JavaScript HTTP 
client library called XMLHttpRequest. Its interface is simple because the browser envi- 
ronment handles the hairy edge cases (proxies, HTTPS, redirects, and so on). Because 
XMLHttpRequest is so simple, and because I want to drive home the point that it's fun- 
damentally no different from (say) Ruby's open- uri, I'm going to cover almost the whole 
interface in this section and the next. If you're already familiar with XMLHttpRequest, 
feel free to skim this section, or skip to the end where there's a nice chart. 

To build an HTTP request you need to create an XMLHttpRequest object. This seemingly 
simple task is actually one of the major points of difference between the web browsers. 
This simple constructor works in Mozilla-family browsers like Firefox: 

request = new XMLHttpRequestQ ; 

The second step is to call the XMLHttpRequest. open method with information about the 
request. All but the first two arguments in this sample call are optional: 

request. open ([HTTP method], [URI], true, [Basic auth username], [Basic auth password]); 

Pretty self-explanatory, except for the third argument, which I've hard-coded to true. 
This argument controls whether the browser carries out the request asynchronously 
(letting the user do other things while it's going on) or synchronously (locking up the 
whole browser until it gets and parses the server response). Locking up the browser 
never creates a good user experience, so I never recommend it, even in simple appli- 
cations. This does mean you have to set up a handler function to be called when the 
request completes: 

request. onReadyStateChange = [Name of handler function]; 

If you want to set any HTTP request headers, you use setrequestHeader: 
request. setRequestHeader([Heac/er name], [Header value]); 

Then you send the request to the HTTP server by calling send. If the request is a POST 
or PUT request, you should pass the entity-body you want to send as an argument to 
send. For all other requests, it should be null. 

request. send([Entity-body]); 

If all goes well, your handler function (the one you set to 
request. onReadyStateChange) will be called four times over the lifetime of the HTTP 



Making the Request | 323 



request, and the value of request. readyState will be different every time. The value 
you're looking for is the last one, 4, which means that the request has completed and 
it's time to manipulate the response. If request. readyState doesn't equal 4, you'll just 
return from the handler function. 

XMLHttpRequest uses the underlying web browser code to make its requests. Since the 
major web browsers are among the most complete HTTP client implementations 
around, this means that XMLHttpRequest does pretty well on the feature matrix I intro- 
duced for HTTP clients back in Chapter 2. Cookies, proxies, and authentication tend 
to work in Ajax applications as they do in normal web access. 

Handling the Response 

Eventually the request will complete and the browser will call your handler function 
for the last time. At this point your XMLHttpRequest instance gains some new and in- 
teresting abilities: 

• The status property contains the numeric status code for the request. 

• The responseXML property contains a preparsed DOM object representing the re- 
sponse document — assuming it was served as XML and the browser can parse it. 
HTML, even XHTML, will not be parsed into responseXML, unless the document 
was served as an XML media type like application/xml or application/xhtmi+xml. 

• The responseText property contains the response document as a raw string — useful 
when it's JSON or some other non-XML format. 

• Passing the name of an HTTP header into the getResponseHeader method looks up 
the value of that header. 

Web browsers epitomize the tree-style parsing strategy that turns a document into a 
data structure. When you make a web service request from within JavaScript, the 
responseXML property gives you your response document as a tree. You can access the 
representation with a standardized set of DOM manipulation methods. 

S 

Unlike the XMLHttpRequest interface, the DOM interface is extremely 
complex and I won't even think about covering it all here. See the official 
*^ fL **,' standard (http://www.w3.org/DOM), the Mozilla DOM reference 
(http://www.mozilla.org/docs/dom/), or a book like Dynamic HTML: The 
Definitive Reference by Danny Goodman (O'Reilly). 

You can navigate the tree with methods like getElementBylD, and run XPath queries 
against it with evaluate. 

But there's another treelike data structure in town: the HTML document being dis- 
played in the end user's web browser. In an Ajax application, this document is your 
user interface. You manipulate it with the same DOM methods you use to extract data 



324 | Chapter 11: Ajax Applications as REST Clients 



from an XML web service representation. An Ajax application acts as glue between the 
raw data the web service sends, and the HTML GUI the end user sees. Useful DOM 
methods here are createTextNode and createElement, both of which I used in Exam- 
ple 11-5. 

JSON 

I covered JSON briefly in Chapter 2. I brought it up again in Chapter 9 as one of my 
recommended representation formats. But since it comes from JavaScript, I want to 
show it in action in the Ajax chapter. 

Example 11-6 shows of an Ajax client for Yahool's image search web service. 

Example 11-6. An Ajax client that calls out to a service that serves JSON representations 

<!D0CTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" 

"http://www.w3.org/TR/REC-html40/transitional.dtd"> 

< ! - -yahoo-ajax.html- -> 

<!--An Ajax application that uses the DSON output feature of Yahoo! 
Web Services- -> 

<html> 

<headxtitle>Davascript Yahoo! DSN</titlex/head> 
<body> 

<hl>Davascript Yahoo! DSON example</hl> 

<p id="message">Loading pictures of baby elephants !</p> 

<div id="images"> 
</div> 

<script type="text/javascript"> 
function formatlmages(result) 
{ 

var images = document. getElementById("images"); 
items = result["ResultSet"]["Result"]; 

document .getElementByld ( "message" ) . f irstChild . textContent = 

items. length + " baby elephant pictures:"; 
for (var i = 0; i < items. length; i++) 

{ 

image = items[i]; 
// Create a link 

var link = document. createElement("a"); 

link . setAttribute( "href " , image [ "ClickUrl" ] ) ; 

// Put a thumbnail image in the link. 

var img = document. createElement("img"); 

var thumbnail = image["Thumbnail"]; 

img . setAttribute( "src" , thumbnail[ "Url" ] ) ; 

img . setAttribute( "width " , thumbnail [ "Width" ] ) ; 

img. setAttribute( "height" , thumbnail[ "Height" ] ) ; 

img . setAttribute( "title" , image [ "Height" ] ) ; 



JSON I 325 



link.appendChild(img); 
images. appendChild (link); 

} 

} 

</script> 

<script type="text/javascript" 
src="http://api. search. yahoo.com/ImageSearchService/Vl/imageSearch 
?appid=restbook&query=baby+elephant&output=json&callback=formatImages" /> 

</body> 
</html> 

If you load this HTML file in your web browser you'll see some cute pictures of baby 
elephants, courtesy of Yahoo! Image Search. What you won't see is a browser security 
warning. The del.icio.us example had to ask the browser if it was OK to make an 
XMLHttpRequest to another domain, and even then the browser imposes strict rules 
about when it is OK. But this Ajax client just makes the web service call. That's because 
it doesn't make the call through XMLHttpRequest. It uses a technique described as Java- 
Script on Demand (JoD). JoD bypasses the browser's security policy by fetching cus- 
tom-generated JavaScript from a web service. Because any JSON data structure is a 
valid JavaScript program, this works especially well with web services that serve JSON 
representations. 

Don't Bogart the Benefits of REST 

It's easy for an Ajax application to take all the advantages of REST for itself, and leave 
none of them for the end user. Gmail is a good example of this. The Gmail Ajax appli- 
cation benefits greatly from its use of an addressable, stateless web service. But in terms 
of user experience, all the end user sees is one constantly changing HTML page. No 
addressability for you! If you want to bookmark a search or a particular email message, 
you need to start off at Gmail's plain HTML interface {https://mail.google.com/mail/? 
ui=html). 

Ordinarily, your browser's back and forward buttons move you back and forth through 
application state. This works because the Web is stateless. But if you start using a typical 
Ajax application, your back button breaks. Clicking it doesn't take you backward in 
application state: it takes you to the page you were on before you started using the Ajax 
application. No statelessness for you! 

The underlying cause is the same thing that gives Ajax applications their polished look. 
Ajax applications disconnect the end user from the HTTP request-response cycle. 
When you visit the URI of an Ajax application, you leave the Web. From that point on 
you're using a GUI application that makes HTTP requests for you, behind the scenes, 
and folds the data back into the GUI. The GUI application just happens to be running 
in the same piece of software you use to browse the Web. But even an Ajax application 
can give its users the benefits of REST, by incorporating them into the user interface. 



326 | Chapter 11: Ajax Applications as REST Clients 



Figure 11-1. An entry point into a web service (that is, a URI), maintained through Ajax 

I'm basically asking you to reinvent some of the features of the web browser within 
your application. 

The best example of this is Google Maps, the application that started the Ajax craze. 
At first glimpse, Google Maps looks about as addressable as Gmail. You visit http:// 
maps.google.com/ and are presented with a large-scale map. You can use Ajax to zoom 
in and navigate to any point on the globe, but the URI in your browser's address bar 
never changes. 

But Google Maps also uses Ajax to maintain a "permalink" for whatever point on the 
globe you're currently at. This URI is kept not in your browser's address bar but in an 
a tag in the HTML document (see Figure 1 1-1) . It represents all the information Google 
Maps needs to identify a section of the globe: latitude, longitude, and map scale. It's a 
new entry point into the Ajax application. This link is the Google Maps equivalent of 
your browser's address bar. 

Thanks to the extra DOM work that keeps this a tag up to date as you navigate the 
map, every point on every map is on the Web. Any point can be bookmarked, blogged 
about, and emailed around. Anyone who visits one of these URIs enters the Google 
Maps Ajax application at the right point, instead of getting a view centered on the 
continental US (as would happen if you navigated to a place on a map and then reloaded 
http://maps.google.com/) . Addressability, destroyed by Ajax but added back by good 
application design, has allowed communities like Google Sightseeing (http:/ 'i 'google 
sightseeing.com/) to grow up around the Google Maps application. 

Your Ajax applications can give statelessness back by reproducing the functionality of 
the browser's back and forward buttons. You don't have to reproduce the browser's 
behavior slavishly. The point is to let the end user move back and forth in his application 
state, instead of having to start from the beginning of a complex operation if he makes 
a mistake or gets lost. 

Cross-Browser Issues and Ajax Libraries 

As always when web browsers are involved, different clients have different levels of 
support for XMLHttpRequest. And as always seems to happen, Internet Explorer is the 
major outlier. This isn't quite fair, because XMLHttpRequest was a Microsoft invention 



Cross-Browser Issues and Ajax Libraries | 327 



and Internet Explorer was the first browser to support Ajax at all. But until the release 
of Internet Explorer 7, Ajax was implemented as Windows-specific technology: an 
ActiveX control called XMLHttp. 

The cross-platform Mozilla project adopted the API of the XMLHttp control, but imple- 
mented it as a class you could instantiate directly from JavaScript. Other browsers 
followed this lead, and all current browsers now use the XMLHttpRequest name (includ- 
ing the new Internet Explorer). But old versions of Internet Explorer still make up a big 
portion of the user base, so cross-browser issues are still a problem. 

Example 11-7 is a JavaScript function that always creates an object that acts like an 
XMLHttpRequest, even though under the covers it may be an ActiveX control. It was 
written by Bret Taylor and comes from his site at http://ajaxcookbook.org/. 

Example 11-7. A cross-browser wrapper for XMLHttpRequest 

function createXMLHttpRequest() { 
if (typeof XMLHttpRequest != "undefined") { 

return new XMLHttpRequestQ; 
} else if (typeof ActiveXObject != "undefined") { 

return new ActiveXObject ( "Microsoft .XMLHTTP" ) ; 
} else { 

throw new Error("XMLHttpRequest not supported"); 

} 

} 

This function is a drop-in replacement for the XMLHttpRequest constructor in Exam- 
ple 11-3, instead of this: 

request = new XMLHttpRequestQ; 

you might write this: 

request = createXMLHttpRequestQ; 

I know of two other major cross-browser issues. First, the Safari browser doesn't sup- 
port the PUT and DELETE methods. If you want your service to be accessible from 
Safari, you'll need to allow your clients to simulate PUT and DELETE requests with 
overloaded POST. Second, Microsoft Internet Explorer caches successful responses 
indefinitely. This makes it look to the user like your resources haven't changed, even 
when they have. The best way to get around this is to send proper ETag response headers 
with your representations, or to disable caching altogether with Cache-Control. You 
can use the XMLHttpRequest test suite (http://www.mnot.net/javascript/xmlhttprequest/) 
to find out about more minor cross-browser quirks. 

Because Ajax is a very important niche for JavaScript applications, some JavaScript 
libraries include wrappers for hiding the differences between browsers. I'm not going 
to cover these frameworks in detail, because they act more as standard libraries for 
JavaScript than tools for building web service clients. I will show how to make simple 
HTTP requests with two popular libraries, Prototype and Dojo. Another popular li- 
brary, script. aculo. us (http://script.aculo.us/), is based on Prototype. 



328 | Chapter 11: Ajax Applications as REST Clients 



Prototype 

Prototype (http://prototype.conio.net/) introduces three classes for making HTTP 
requests: 

• Ajax. Request: a wrapper around XMLHttpRequest that takes care of cross-browser 
issues and can call different JavaScript functions on the request's success or failure. 
The actual XMLHttpRequest object is available as the transport member of the 
Request object, so responseXML will be through request. transport. responseXML. 

• Ajax.Updater: a subclass of Request that makes an HTTP request and inserts the 
response document into a specified element of the DOM. 

• Ajax.PeriodicalUpdater, which makes the same HTTP request at intervals, re- 
freshing a DOM element each time. 

I've implemented the del.icio.us Ajax client in Prototype, and it was mostly the same 
as the client I showed you starting in Example 11-1. The code snippet below mostly 
replaces the code in Example 11-3 where the XMLHttpRequest constructor used to be. 
Note the new script tag, the use of request. transport instead of request, and the use 
of Prototype's onFailure hook to signal a failure (such as an authorization failure) to 
the user. 

Example 11-8. A portion of ajax-delicious-prototype.html 

<script src="prototype. js"x/script> 
<script type="text/javascript"> 

var request = new Ajax. Request("https://api. del. icio.us/vl/posts/recent", 

{method: 'get', onSuccess: populateLinkList, 
onFailure: reportFailure}); 

■function reportFailureQ { 

setMessage("An error occured: " + request. transport. status); 

} 

// Called when the HTTP request has completed, 
function populateLinkListQ { 
setMessage("Request complete. "); 

if (netscape. security. PrivilegeManager.enablePrivilege) { 
netscape. security. PrivilegeManager. enablePrivilege( "UniversalBrowserRead" ) ; 

} 

posts = request .transport . responseXML . getElementsByTagName( "post" ) ; 

In its quest to simplify XMLHttpRequest, Prototype hides some of the features. You can't 
set request headers, or specify a username and password for basic HTTP auth. So even 
if you're using Prototype, you might want to keep around a snippet of code like the one 
in Example 11-7. On the other hand, the Prototype implementation of the del.icio.us 
client doesn't need the username and password text fields at all: it just needs a button. 



Cross-Browser Issues and Ajax Libraries | 329 



The end user's browser will prompt her anyway for her del.icio.us username and 
password. 

Dojo 

The Dojo library (http://dojotoolkit.org/) provides a uniform API that not only hides the 
differences between browsers when it comes to XMLHttpRequest, it hides the difference 
between XMLHttpRequest and other ways of getting the browser to send an HTTP re- 
quest. These "transports" include tricks that use HTML tags, such as JoD. All the 
variants on XMLHttpRequest are kept in the dojo.io.XMLHttp transport class. For all 
transports, the bind method is the one that makes the HTTP request. 

As with Prototype, I've implemented the del.icio.us Ajax client with Dojo, and it's 
mostly the same as the original, except for the section in Example 11-3 where the 
XMLHttpRequest constructor used to be. Example 11-9 shows the relevant portions of 
ajax-delicious-dojo.html. 

Example 11-9. Some portions of ajax-delicious-dojo.html 

<script src="dojo/dojo. js"x/script> 
<script type="text/ javascript "> 

dojo. require( "dojo. io.*"); 

dojo.io.bind({ url: "https://api.del.icio.us/vl/posts/recent", load: 
populateLinkList, error: reportFailure }); 

function reportFailure(type, error) { 
setMessage("An error occured: " + error. message); 

} 

// Called when the HTTP request has completed, 
function populateLinkList(type, data, request) { 
setMessage("Request complete. "); 

if (netscape. security. PrivilegeManager.enablePrivilege) { 
netscape . security . PrivilegeManager . enablePrivilege( "UniversalBrowserRead " ) ; 

} 

posts = request. responseXML.getElementsByTagName("post"); 

The error-handling function is passed a dojo. io. Error object with members called 
number and message. You can ignore the first argument: it's always "error." You can also 
ignore the first argument to the success-handling function (it's always "load"). The 
second argument, called data above, is an interface to use Dojo's DOM manipulation 
interface. If you want to use the XMLHttpRequest interface instead, you can ignore that 
argument too. 



330 | Chapter 11: Ajax Applications as REST Clients 



Subverting the Browser Security Model 

That's a provocative title but I stand by it. A web browser enforces a general rule that's 
supposed to prevent it from using code found on domain A to make an HTTP request 
to domain B. I think this rule is too strict, so I'm going to show you two ways around 
it: request proxying and JoD. I'm also going to show how these tricks put you at risk 
by making you, the Ajax programmer, accept responsibility for what some foreign 
server does. These tricks deserve to be regarded as cheats, because they subvert rather 
than fulfill the web browser's intentions. They often make the end user less secure than 
if his browser had simply allowed domain A's JavaScript to make an HTTP request to 
domain B. 

There is a secure method of getting permission to make foreign web service calls in your 
JavaScript applications, which is to ask for the permission by calling: 

netscape. security. PrivilegeManager. enablePrivilege( "UniversalBrowserRead" ) ; 

(There's also an insecure method, which is to have your users use Internet Explorer 
with the security settings turned way down.) 

If your script is digitally signed, the client's browser shows your credentials to the end 
user. The end user makes a decision whether or not to trust you, and if he trusts you 
he gives you permission to make the web service calls you need to make. This is similar 
to the technique I mentioned in Chapter 8, where an untrusted web service client was 
trying to gain the end user's trust. The difference here is that the untrusted web service 
client is running inside the end user's trusted web browser. 

There are two problems with the secure method. The first is that, as you might have 
guessed from the name netscape, security. PrivilegeManager, it only works in Mozilla, 
Firefox, and Netscape-like browsers. The second is that it's quite painful to actually 
get a signed script set up. Once you do get one set up, you find you've stored your 
HTML files in a signed Java archive file, and that your application is off the Web! Search 
engines won't pick up your HTML pages, and you'll only be able to address them 
through a weird jar: URIlike jar: http://www.example.com/ajax-app.jar! /index, html. 

And that's the right solution. As you can tell, this is an immature field. Until recently, 
web services weren't popular enough for people to seriously think about these prob- 
lems. Though the hacks described below are potentially dangerous, their inventors 
meant no harm. They were motivated only by zeal for the enormous possibilities of in- 
browser web service clients. The challenge is to come up with ways of getting the same 
functionality without sacrificing security, adding too much complexity, or moving Ajax 
applications out of view of the Web. The W3C is working on this problem (see "Ena- 
bling Read Access for Web Resources" at http://www.w3.org/TR/access-control/.) 

Although I'm focusing again on JavaScript applications, Java applets and Flash also 
run under security models that prevent them from sending data to foreign servers. The 
request proxying trick, described below, works for any kind of Ajax application, be- 



Subverting the Browser Security Model | 331 



cause it involves work on the server side. As its name implies, the JoD trick is JavaScript- 
specific. 

Request Proxying 

You're running a site, example.com, serving up Ajax applications that try to make 
XMLHttpRequest requests against yahoo.com. Naturally your clients' web browsers will 
complain. But what if they never made a request to yahoo.com? What if they made 
requests to example . com, which you handled by making your own, identical requests to 
yahoo.com without telling the client? 

Welcome to the request proxy trick, well described in Yahoo's document "Use a Web 
Proxy for Cross-Domain XMLHttpRequest Calls" (http://developer.yahoo.com/java 
script/howto-proxy. html) . In this trick, you set aside part of the URI space on your server 
to simulate the URI space on some other server. When you get a request to a URI in 
that space, you send it along without alteration to the foreign server, and then pipe the 
response right back to the client. From the client's point of view, it looks like you're 
providing someone else's web service. Really, you're just filing the domain names off 
their HTTP responses and replacing them with your own. 

If you're using Apache and have mod_proxy installed, the simplest way to set up a 
proxy is in the Apache configuration. If you also have mod_ssl installed, you can enable 
SSLProxyEngine and proxy HTTPS requests. So long as you have mod_ssl installed, you 
can even proxy HTTPS requests from an HTTP server: perhaps http://example.com/ 
service/ is proxied to https://service.com/. Of course, this destroys the security of the 
connection. Data is secure between the proxy and your site, but not between your site 
and the end user. If you do this you'd better tell the end user what you're doing. 

Let's say you want to make the del.icio.us Ajax application, given above, work from 
your site at example.com. You can set up a proxy so that all URIs beneath https:llexa.m- 
ple.com/apis/delicious/vl/ are transparently forwarded to https://api.del.icio.us/vl/ . The 
simplest way to set up a proxy is with the ProxyPass directive, which maps part of your 
URI space onto a foreign site's URI space (see Example 11-10). 

Example 11-10. Apache configuration with ProxyPass 
SSLProxyEngine On 

ProxyRequests Off # Don't act as an open proxy. 
ProxyPass /apis/delicious/vl https://api.del.icio.us/vl/ 

A more flexible solution is to use a rewrite rule with the [P] flag. This gives you the full 
power of regular expressions to map your URI-space onto the foreign site's. Exam- 
ple 11-11 shows a rewrite rule version of the del.icio.us API proxy: 

Example 11-11. Apache configuration with rewrite rules 
SSLProxyEngine On 

ProxyRequests Off # Don't act as an open proxy. 

RewriteEngine On RewriteRule A apis/delicious/vl/(.*)$ https://api.del.icio.us/vl/Sl [P] 



332 | Chapter 11: Ajax Applications as REST Clients 



With a setup like one of those two, you can serve the Ajax application delicious- 
ajax.html from your own domain, without triggering browser security warnings. All 
you have to do is change this (from Example 11-4): 

request. open( "GET", "https://api.del.icio.us/vl/posts/recent", 
true, username, password); 

to this: 

request. open( "GET", "https://example.com/apis/delicious/vl/posts/recent", 
true, username, password); 

Most Apache installations don't have mod_proxy installed, because an open HTTP 
proxy is a favorite tool for spammers and other lowlife who want to hide their tracks 
online. If your web server doesn't have built-in proxy support, you can write a tiny web 
service that acts as a transparent proxy, and run it on your server. To proxy del.icio.us 
API requests, this web service might be rooted at apis/delicious/vl. It would pass any 
and all HTTP requests it received — HTTP headers and all — to the corresponding URI 
beneath https://api.del.icio.us/vl/ '. Yahoo! provides a sample proxy service, written in 
PHP, hardcoded to access the yahoo.com web services (http://developer.yahoo.com/java 
script/howto-proxy.html). You can model your own proxy service after that one. 

Even when your proxy is properly configured, when it only proxies requests for a very 
small subset of the Web, there is danger for you and your end users. When you set up 
a proxy for Ajax clients, you're taking responsibility in your users' eyes for what the 
other web site does. The proxy trick sets you up as the fall guy for anything bad that 
happens on the other site. You're pretending what the other site is serving comes from 
you. If the web service crashes, cheats the end user, or misuses his personal data, guess 
what: it looks like you did those things. Remember, in an Ajax application the end user 
only sees your GUI interface. He doesn't necessarily know his browser is making HTTP 
requests in the background, and he certainly doesn't know that his requests to your 
domain are being proxied to another domain. If his web browser knew that was going 
on, it would step in and put a stop to it. 

The proxy trick also sets you up as the fall guy for the requests your clients make. Your 
clients can make any web service request they like and it'll look like you're the cause. 
Depending on the nature of the web service this may cause you embarrassment or legal 
exposure. This is less of a problem for web services that require separate authorization. 

JavaScript on Demand 

It's rare for a human being to demand JavaScript, except in certain design meetings, 
but it's not uncommon among web browsers. The basis of this trick is that the HTML 
script tag doesn't have to contain hardcoded JavaScript code. It might just have a 
src attribute that references code at another URI. A web browser knows, when it en- 
counters a script tag, to load the URI in the src attribute and run its contents as code. 



Subverting the Browser Security Model | 333 



We saw this in Example 11-6, the JSON example that does a Yahoo! image search for 
pictures of elephants. 

The src attribute is traditionally used like C's #include or Ruby's require: to load in a 
JavaScript library from another URL Example 11-12, reprinted from Chapter 2, shows 
this. 

Example 11-12. Including a JavaScript file by reference 

<!-- In a real application, you would save json.js locally 

instead of fetching it from json.org every time. --> 
<script type="text/javascript" src="http : //www. json.org/json. js"x/script> 

As you can see, the URI in the src attribute doesn't have to be on the same server as 
the original HTML file. The browser security model doesn't consider this insecure 
because... well, near as I can figure, because the src attribute was already in wide use 
before anyone started seriously thinking about the security implications. 

Now cast your mind back to the elephant example in Example 1 1-6. It includes this line: 
<script type="text/javascript" 

src="http : //api . search .yahoo . com/ImageSearchService/Vl/ imageSearch 
?appid=restbook&query=baby+elephant&output=json&callback=formatImages" /> 

That big long URI doesn't resolve to a standalone JavaScript library, the way http:// 
www.json.org/json.js does. If you visit it in your web browser you'll see that URI's rep- 
resentation is a custom- generated bit of JavaScript. In its developer documentation 
(http://developer.yahoo.com/common/json.html), Yahoo! promises that the representa- 
tion of a resource like this one is a snippet of JavaScript code. Specifically, a snippet of 
JavaScript code that passes a data structure as the only argument into a callback func- 
tion named in the URI (here, it's f ormatlmages) . The resulting JavaScript representation 
looks something like this: 

formatImage({"ResultSet" :{"totalResultsAvailable" : "27170", ...}}) 

When the client loads the HTML page, it fetches that URI and run its body as JavaScript, 
incidentally calling the f ormatlmage method. Great for our application; not so great for 
the web browser. From a security perspective this is just like JavaScript code that uses 
XMLHttpRequest to get data from the Yahoo! web service, and then calls f ormatlmage on 
the result. It bypasses the browser security model by making the HTTP request happen 
as a side effect of the browser's handling of an HTML tag. 

JoD switches the traditional roles of a script embedded in an HTML page and a script 
included via <script src=" ...">. Your web browser requests a web service URI, think- 
ing it's just a JavaScript library that application code in the HTML page will eventually 
call. But the library function is the one defined locally (it's formatlmage), and the ap- 
plication code that calls that function is coming from a foreign site. 

If you specify no callback in the URI when calling the Yahoo! web service, you get a 
"JavaScript" file containing nothing but a JSON data structure. Including this file in a 
script tag won't do anything, but you can fetch it with a programmable HTTP client 



334 | Chapter 11: Ajax Applications as REST Clients 



(like XHLHttpRequest, or the Ruby client from way back in Example 2-15) and parse it 
as data: 

{"ResultSet":{"totalResultsAvailable": "27170", . . .}} 

Dynamically writing the script tag 

The only example of JoD I've given so far has a hardcoded script tag. The URI to the 
web service resource is fixed in stone, and if the end user wants to see baby penguins 
instead of baby elephants he's just out of luck. 

But one of the things you can do with JavaScript is add brand new tags to the DOM 
object representing the current HTML page. And script is just another HTML tag. 
You can use JavaScript to write a customized script tag into the document, and get the 
browser to load the URI mentioned in its src attribute as a side effect of the script 
processing. The browser allows this even if the src URI points to a foreign domain. 
That means you can use JavaScript to make requests to any URI that serves more Java- 
Script, and run it. 

This works, but it's a hack on top of a hack, and a security problem on top of a security 
problem. In fact, from a security perspective this is worse than using XMLHttpRequest to 
get data from a foreign site. The worst XHLHttpRequest will do is make an HTTP request 
and parse some XML into a tree-like data structure. With JoD you make an HTTP 
request and run previously unseen JavaScript code as though it was part of your original 
program. 

You and your end user are completely at the mercy of the service you're calling. Instead 
of JavaScript that does what you want, a malicious web service might decide to serve 
JavaScript that steals whatever cookies your domain set for this user. It might serve 
code that runs code as promised but also creates pop-up windows full of obnoxious 
ads. It might do anything at all. And since Ajax hides the HTTP request-response cycle 
from the end user, it looks like your site is responsible! 

Now, maybe you trust a brand-name site like Yahoo! (unless it gets cracked), but you 
probably don't trust Mallory's House of Web Services. And that in itself is a problem. 
One of the nice things about the Web is that you can safely link to Mallory even if you 
don't trust her, don't have her permission, and think she's wrong about everything. A 
normal web service client can make calls to Mallory's web service, and examine the 
representation before acting on it in case she tries any trickery. But when the client is 
serving executable code, and the web service requested it through a hack that runs the 
code automatically, you're reduced to operating on blind trust. 

JoD is not only sketchy from a security standpoint, it's a lousy tactic from a REST 
standpoint, because it forces you to use a crippled client. XMLHttpRequest supports all 
the features of HTTP, but with JoD you can only make GET requests. You can't send 
request headers, see the response code or headers, or handle representation formats 
other than JavaScript code. Any representation you receive is immediately executed as 
JavaScript. 



Subverting the Browser Security Model | 335 



The underlying technique, of referencing a new object in a src attribute, is safer when 
you use it to grab resources other than custom-generated JavaScript, script isn't the 
only HTML tag that makes the browser load a representation. Other useful tags include 
img and frame. Google Maps uses img tags rather than XMLHttpRequest calls to fetch its 
map tile images. Google's JavaScript code doesn't make the HTTP requests. It just 
creates the img tags and lets the browser make requests for the images as a side effect. 

Library support 

Jason Levitt has written a JavaScript class called DSONscriptRequest that makes JoD 
easy (http://www.xml.eom/pub/a/2005/12/21/json-dynamic-script-tag.html). This class 
works sort of like XMLHttpRequest, except it supports fewer of HTTP's features, and 
instead of expecting the server to send an XML representation, it expects a snippet of 
JavaScript. 

Example 11-13 shows a dynamic implementation of the image search Ajax application. 
The first part should be familiar if you've looked at the other Ajax applications in this 
chapter. 

Example 11-13. Dynamic Yahoo! image search Ajax application 

<!D0CTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" 

"http://www.w3.org/TR/REC-html40/transitional.dtd"> 

< ! --yahoo-a jax-dynamic.html- -> 

<!--An Ajax application that uses dynamic SCRIPT tags to interactively 
fetch data from Yahoo! Web Services and call predefined functions 
on that data.--> 

<html> 

<headxtitle>Davascript Yahoo! DSON - Dynamic</titlex/head> 
<body> 

<hl>Davascript Yahoo! DSON example with dynamic SCRIPT tags</hl> 
<form onsubmit="callYahoo() ; return false; "> 

What would you like to see? <input id="query" type="text" /xbr /> 
<input type="submit" value="Fetch pictures from Yahoo! Image Search"/> 
</form> 

<div id="images"> 
</div> 

< script type="text/javascript"> 
function formatlmages(result) 
{ 

// First clear out any old images. 

var images = document. getElementById("images"); 

while (images. firstChild) 

{ 

images . removeChild (images . firstChild) ; 

} 

items = resultf'ResultSet"] ["Result"]; 
for (var i = 0; i < items. length; i++) 



336 I Chapter 11: Ajax Applications as REST Clients 



{ 

image = items[i]; 



// Create a link 

var link = document. createElement("a"); 

link . setAttribute( "href " , image [ "ClickUrl" ] ) ; 

// Put a thumbnail image in the link. 

var img = document. createElement("img"); 

var thumbnail = image["Thumbnail"]; 

img . setAttribute( "src" , thumbnail [ "Url" ] ) ; 

img . setAttribute( "width " , thumbnail [ "Width" ] ) ; 

img. setAttribute( "height" , thumbnail [ "Height" ] ) ; 

img. setAttribute( "title", image["Height"]); 

link. appendChild( img); 

images. appendChild (link); 

} 

} 

</script> 

Here's where this application diverges from others. I include Jason Levitt's 
jsr_class.js file, and then define the callYahoo function to use it (see Example 11-14). 
This is the function triggered when the end user clicks the submit button in the HTML 
form above. 

Example 11-14. Dynamic Yahoo! image search Ajax application continued 
<script type=" text/ javascript" src=" jsr_class. js"x/script> 

<script type="text/javascript"> 

function callYahoo() 

{ 

var query = document. getElementById("query"). value; 

var uri = "http://api.search.yahoo.com/ImageSearchService/Vl/imageSearch" + 
"?query=" + escape(query) + 
"&appid=restbook&output=json&callback=formatImages"; 
alert(uri); 

var request = new DSONscriptRequest(uri); 
request. buildScriptTag(); 
request. addScriptTag(); 

} 

</script> 

</body> 
</html> 

To make a web service request I pass the URI of a resource into a DSONscriptRequest 
object. The addScriptTag method sticks a new script tag into the DOM. When the 
browser processes its new tag, it makes a GET request to the foreign URI, and runs the 
JavaScript that's served as a representation. I specified "callback=formatlmages" in the 
URI's query string, so Yahoo! serves some JavaScript that calls my formatlmages func- 
tion on a complex data structure. You can serve this Ajax application from anywhere, 



Subverting the Browser Security Model | 337 



and use it to search for anything on Yahoo !'s image search, without triggering any 
browser warnings. 

The Dojo library makes the script trick easy by providing a dojo.io.SrcScript trans- 
port class that uses it. It also provides a dojo.io.IframelO class which uses a similar 
trick involving the iframe tag. This trick also requires cooperation from the server, but 
it does have the advantage that it doesn't automatically execute the response document 
as code. 



338 | Chapter 11: Ajax Applications as REST Clients 



CHAPTER 12 



Frameworks for RESTful Services 



As the REST design philosophy becomes more popular, new frameworks are springing 
up to make RESTful design easy. Existing frameworks are acquiring RESTful modes 
and features. This, in turn, drives additional interest in REST. In this chapter, I and a 
few knowledgeable contributors show you how to write resource-oriented services in 
three popular frameworks: Ruby on Rails, Restlet (for Java), and Django (for Python). 

Back in Chapter 1 I said that REST isn't an architecture, but a way of judging archi- 
tectures. The Resource-Oriented Architecture is an architecture: it imposes constraints 
on your thinking that make it easy for you to break a problem down into RESTful 
resources. But these resources still only exist on an abstract level. They aren't real until 
you expose them through specific web services. 

If you're writing a service from scratch (say, as a CGI script), you can translate your 
resources into code however you like. But most services aren't written from scratch: 
they're written using a web framework. A REST-aware web framework imposes con- 
straints on your programming that make it easy for you to implement RESTful resources 
in a specific programming language. In this chapter I'll show you how to integrate the 
lessons of this book with real frameworks. 

Ruby on Rails 

The simplifying assumption is the main driver of the success of Ruby on Rails. Rather 
than give you a large number of tools for accomplishing any task you can think of, Rails 
gives you one way to accomplish a wide variety of common tasks. You can create a 
Rails application very quickly if you're trying to expose data from a relational database, 
if your database tables have certain names and structure, if you care to work with a 
Model- View-Controller architecture, and so on. Because so many problems in the web 
application domain fit these assumptions, the effect is rarely onerous and often liber- 
ating. 

Earlier versions of Rails exposed a textbook REST-RPC hybrid architecture, but Rails 
1.2 focuses on a more RESTful design. Perhaps this was inevitable: HTTP's uniform 



339 



interface is just another simplifying assumption. I've already shown in Chapter 7 how 
Rails can be used to make sophisticated RESTful services in very little code. In this 
section, I take a step back and describe the RESTful architecture of Rails in more general 
terms. 

Routing 

When an HTTP request comes in, Rails analyzes the requested URI and routes the 
request to the appropriate controller class. As shown in Example 12-1, the file config/ 
routes.rb tells Rails how to handle certain requests. 

Example 12-1. A simple routes.rb file 
# routes.rb 

ActionController : : Routing : : Routes . draw do | map | 

map. resources :weblogs do |weblog| 
weblog. resources : entries 

end 
end 

A config/routes.rb file can get fairly sophisticated. The one in Chapter 7 is relatively 
complex: I had a lot of resources, and I had to fight the simplifying assumptions a little 
bit to get the URI structure I wanted. Example 12-1 shows a simpler routes.rb file that 
buys into the simplifying assumptions. 

That file declares the existence of two controller classes (WeblogsController and 
EntriesController), and tells Rails how to route incoming requests to those classes. 
WeblogsController handles requests for the URI /weblogs, and for all URIs of the 
form /weblogs/{id}. When present, the path variable {id} is made available as par 
ams[ :id]. 

EntriesController handles requests for the URI /weblogs/{weblog_id}/entries, and all 
URIs of the form /weblogs/{weblog_id}/entries/{id}. The path variable {weblog_id} 
is made available as params[ :weblog_id], and {id}, if present, is made available as 
params[:id]. 

Variables like {id} and {weblog_id} are typically used to associate a resource with a 
particular object in the system. They often correspond to database IDs, and get plugged 
into the ActiveRecord find method. In my del.icio.us clone I tried to give them de- 
scriptive names like {username}, and used them as identifying names rather than IDs. 

Resources, Controllers, and Views 

As I showed in Chapter 7, every Rails controller might expose two kinds of resources. 
You can have a single "list" or "factory" resource, which responds to GET and/or POST 
requests, and you can have a large number of "object" resources, which respond to 
GET, POST, and/or DELETE. The list resource often corresponds to a database table, 
and the object resources to the rows in the table. 



340 | Chapter 12: Frameworks for RESTful Services 



Each controller is a Ruby class, so "sending" an HTTP request to a class means calling 
some particular method. Rails defines five standard methods per controller, as well as 
exposing two special view templates through HTTP GET. For illustration's sake, here 
are the seven HTTP requests made possible by my call to map. resources :weblogs back 
in Example 12-1: 

• GET /weblogs: A list of the weblogs. Rails calls the WeblogsControllerttindex 
method. 

• GET /weblogs/new: The form for creating a new weblog. Rails renders the view in 
app/view/welogs/new.rhtml. This view is a hypermedia file describing what sort of 
HTTP request the client must make to create a new weblog. 

In other words, this is an HTML form (though it could also be a small WADL file) . 
The form says that to create a new weblog, the client should send a POST request 
to /weblogs (see below). It also tells the client how to format its representation of 
the new weblog, so that the server can understand it. 

• POST /weblogs: Create a new weblog. Rails calls the WeblogsController#create 
method. 

• GET /weblogs/{id}: A weblog. Rails calls WeblogsController#show. 

• GET /weblogs/{id};edit: The form for editing a Weblog's state. Rails renders the 
view in app/view/welogs/edit.rhtml. This view is a hypermedia file describing what 
sort of HTTP request the client must make if it wants to edit a weblog's state. 

In practice, this means the view is an HTML form, or short WADL file. The hy- 
permedia file tells the client how to send or simulate a PUT request to /weblogs/{id}. 

• PUT /weblogs/{id}: Change a weblog's state. Rails calls 
WeblogsController#update. The "state" here is the state associated with this par- 
ticular resource: things like the weblog's name and the author's contact informa- 
tion. Individual entries are exposed as separate resources. 

• DELETE /weblogs/{id}: Delete a weblog. Rails calls WeblogsController#delete. 

You probably won't expose all seven access points in every controller you create. In 
particular, you probably won't use the special views unless you're running your web 
service as a web site. This is no problem: just don't implement the methods or view 
files you don't intend to expose. 

Outgoing Representations 

Rails makes it easy to send different representations of a resource based on the client's 
request. Example 12-2 shows some hypothetical Ruby code that renders three different 
representations of a weblog. Which representation is sent depends on the URI the client 
accessed, or on the value it provided in the Accept header. A client will get the HTML 
rendition if it accesses /weblogs/l.html, but if the client accesses /weblogs/l.png 
instead, the service will send a graphical PNG rendition. The respondjto function takes 



Ruby on Rails | 341 



care of interpreting the client's capabilities and desires. All you have to do is implement 
the supported options, in order of precedence. 

Example 12-2. Serving one of several representations 

respond_to do | format | 
format.html { render : template => 'weblogs/show' } 
format. xml { render :xml => weblog.to_xml } 
format. png { render :text => weblog.generate_image, 
:content_type => "image/png" } 

end 

Two especially common representation formats are HTML and the ActiveResource 
XML serialization format. HTML representations are expressed using Rails views, as 
they would be in a human-oriented web application. To expose an ActiveRecord object 
as an XML document, you can just call to_xml on an object or a list of objects. 

Rails plugins make it easy to expose data in other representation formats. In Chap- 
ter 7, 1 installed the atom-tools Ruby gem so that I could render lists of bookmarks as 
Atom feeds. In Example 7-8 I have a respond_to code block, containing clauses which 
distinguish between requests for Atom and generic XML representations. 

Incoming Representations 

Rails sees its job as turning an incoming representation into a bunch of key-value pairs, 
and making those key-value pairs available through the params hash. By default, it 
knows how to parse form-encoded documents of the sort sent by web browsers, and 
simple XML documents like the ones generated by to_xml. 

If you want to get this kind of action for your own incoming representations, you can 
add a new Proc object to ActionController : : Base . param_parsers hash. The Proc object 
is a block of code whose job is to process an incoming representation of a certain media 
type. For details, see the Rails documentation for the param_parsers hash. 

Web Applications as Web Services 

Rails 1.2 does an excellent job of merging the human web and the programmable web. 
As I showed in Chapter 3, Rails comes with a code generator called scaf 
fold_resource which exposes a database table as a set of resources. You can access the 
resources with a web browser, or with a web service client like ActiveResource. 

If you use a web browser to access a scaf f old_resource service, you're served HTML 
representations of the database objects, and HTML forms for manipulating them (gen- 
erated by the new.rhtml and edit.rhtml I mentioned earlier). You can create, modify, 
and delete the resources by sending new representations in form-encoded format. PUT 
and DELETE requests are simulated through overloaded POST. 



342 | Chapter 12: Frameworks for RESTful Services 



If you use a web service client to access a scaf f old_resource service, you're served XML 
representations of the database objects. You manipulate objects by modifying the XML 
documents and sending then back with PUT. Non-overloaded POST and DELETE 
work like you'd expect. 

There's no more compelling example of the human web's basic similarity to the pro- 
grammable web. In Chapter 7 I largely ignored this aspect of Rails for space reasons, 
but it makes a compelling argument for using Rails if you're designing a web site and 
a web service to do the same thing. Rails makes it easy to expose them both as aspects 
of the same underlying code. 

The Rails/ROA Design Procedure 

The following list is a modified version of the generic design procedure from Chap- 
ter 6. It's what I used, unofficially, to design the service in Chapter 7. The main differ- 
ence is that you divide the dataset into controllers and the controllers into resources, 
rather than dividing the dataset into resources. This reduces the chance that you'll end 
up with resources that don't fit Rails's controller system. 

1. Figure out the dataset. 

2. Assign the dataset to controllers. 
For each controller: 

a. Does this controller expose a list or factory resource? 

b. Does this controller expose a set of object resources? 

c. Does this controller expose a creation form or editing form resource? 
For the list and object resources: 

• Design the representation (s) accepted from the client, if different from the 
Rails standard. 

• Design the representation (s) served to the client. 

• Connect this resource to existing resources. 

• Consider the typical course of events: what's supposed to happen? The 
database-backed control flow from Chapter 9 should help here. 

• Consider error conditions: what might go wrong? Again, you can often use 
the database-backed control flow. 



Restlet 

by Jerome Louvel and Dave Pawson 

The Restlet project (http://www.restlet.org) provides a lightweight but comprehensive 
framework for mapping REST concepts to Java classes. It can be used to implement 



Restlet | 343 



any kind of RESTful system, not just RESTful web services, and it's proven a reliable 
piece of software since its inception in 2005. 

The Restlet project was influenced by the other major Java technologies for developing 
Web applications: the Servlet API, Java Server Pages, HttpURLConnection, and Struts. 
The primary goal of the project is to provide the same level of functionality while stick- 
ing closer to the goals of REST as expounded in the Fielding thesis. Another key goal 
is to present a unified view of the Web, suitable for use in both client- and server-side 
applications. 

The Restlet philosophy is that the distinction between HTTP client and HTTP server 
is architecturally unimportant. A single piece of software should be able to act as a web 
client, then as a web server, without using two completely different APIs.* 

An early development was the split of the software into the Restlet API and Noelios 
Restlet Engine (NRE), a reference implementation. This separation allows other im- 
plementations to be compatible with the same API. The NRE includes several HTTP 
server connectors based on popular HTTP open source Java projects: Mortbay's Jet- 
ty, Codehaus's AsyncWeb, and the Simple framework. There's even an adapter that 
lets you deploy a Restlet application inside standard Servlet containers like Apache 
Tomcat. 

Restlet also provides two HTTP client connectors, one based on the official HttpURL 
Connection class and the other on Apache's popular HTTP client library. Another con- 
nector allows you to easily manipulate a JDBC source via XML documents in a RESTful 
way, while an SMTP connector, based on the JavaMail API, lets you send email with 
an XML document. 

The Restlet API includes classes that can build representations based on strings, files, 
streams, channels, and XML documents: it supports SAX and DOM for parsing, and 
XSLT for transformation. It's easy to build JSP-style template-based representations, 
using the FreeMarker or Apache Velocity template engines. You can even serve static 
files and directories, like an ordinary web server, using a Directory class, which sup- 
ports content negotiation. 

Throughout the framework, the design principles are simplicity and flexibility. The API 
aims to abstract the concepts of HTTP, URIs, and REST into a consistent set of classes, 
without fully hiding low-level information such as the raw HTTP headers. 

Basic Concepts 

The Restlet terminology matches the terminology of REST as described in the Fielding 
thesis: resource, representation, connector, component, media type, language, and so 

Here Restlet follows Benjamin Carlyle's sound advice (http://soundadvice.id.aU/blog/2005/ll/12/#httpAPI). 
Carlyle points out a flaw in the standard Java API: that "the HttpURLConnection class itself looks nothing like 
a servlet." 



344 | Chapter 12: Frameworks for RESTful Services 



Uniform 



I 



Restlet 



Connector 



Application 



Router 



m 



Finder 



Component 



Filter 



Client 


Server 







t 


Redirect 


t 


VirtualHost 




Directory 









Guard 


Route 







Figure 12-1. The Restlet class hierarchy 

on. A lot of this terminology should be familiar to you from elsewhere in the book. 
Restlet adds some specialized classes like Application, Filter, Finder, Router, and 
Route, to make it easier to combine restlets with each other, and to map incoming 
requests to the resources that ought to handle them. 

The central concept of Restlet is the abstract Uniform class, and its concrete subclass 
Restlet. As the name implies, Uniform exposes a uniform interface as defined by REST. 
This interface is inspired by HTTP's uniform interface but can be used with other 
protocols like FTP and SMTP. 

The main method is handle, which takes two arguments: Request and Response. As you 
can see from Figure 12-1, every call handler that is exposed over the network (whether 
as client or server) is a subclass of Restlet — is a restlet — and respects this uniform 
interface. Because of this uniform interface, restlets can be combined in very sophisti- 
cated ways. 

Every protocol that Restlet supports is exposed through the handle method. This means 
HTTP (server and client), HTTPS, and SMTP, as well as JDBC, the file system, and 
even the class loaders all go through handle. This reduces the number of APIs the de- 
veloper must learn. 

Filtering, security, data transformation, and routing are handled by chaining together 
subclasses of Restlet. Filters can provide processing before or after the handling of a 
call by the next restlet. Filter instances work like Rails filters, but they respond to the 
same handle method as the other Restlet classes, not to a filter-specific API. 

A Router restlet has a number of Restlet objects attached to it, and routes each incoming 
protocol call to the appropriate Restlet handler. Routing is typically done on some 
aspect of the target URI, as in Rails. Unlike Rails, Restlet imposes no URI conventions 



Restlet | 345 



on your resource hierarchies. You can set up your URIs however you want, so long as 
you program your Routers appropriately. 

Routers can stretch beyond this common usage. You can use a Router to proxy calls 
with dynamic load balancing between several remote machines! Even a setup as com- 
plex as this still responds to Restlet's uniform interface, and can be used as a component 
in a larger routing system. The VirtualHost class (a subclass of Router) makes it possible 
to host several applications under several domain names on the same physical machine. 
Traditionally, to get this kind of feature you've had to bring in a front-end web server 
like Apache's httpd. With Restlet, it's just another Router that responds to the uniform 
interface. 

An Application object can manage a portable set of restlets and provide common serv- 
ices. A "service" might be the transparent decoding of compressed requests, or tun- 
nelling methods like PUT and DELETE over overloaded POST using the method query 
parameter. Finally, Component objects can contain and orchestrate a set of Connectors, 
VirtualHosts, and Applications that can be run as a standalone Java application, or 
embedded in a larger system such as a J2EE environment. 

In Chapter 6 you saw a sequence of steps for breaking a problem down into a set of 
resources that respond to HTTP's uniform interface. This procedure was modified in 
Chapter 7 to deal with the simplifying assumptions imposed by Ruby on Rails. There's 
no need to modify the procedure when working with Restlet, because Restlet makes 
no simplifying assumptions. It can implement any RESTful system. If you happen to 
be implementing a RESTful resource-oriented web service, you can arrange and im- 
plement the resources however you like. Restlet does provide some classes that make 
it easy to create resource-oriented applications. Most notably, there's a Resource class 
that can be used as the basis for all of your application resources. 

Throughout this book, URI Templates are used as shorthand to designate whole classes 
of URIs (see Chapter 9"). Restlet uses URI Templates to map URIs onto resources. A 
Restlet implementation of the Chapter 7 social bookmarking application might specify 
the path to a particular bookmark like so: 

/users/{username}/bookmarks/{URI} 

You can use this exact syntax when attaching a Resource subclass to a Router. If it sounds 
too good to be true, just wait for the next section where I actually implement part of 
the bookmarking service covered in Chapter 7. 

Writing Restlet Clients 

In Example 2-1 you saw a Ruby client that retrieved XML search results from Yahool's 
web search service. The code in Example 12-3 is a Java implementation of the same 
client, written against version 1 .0 of Restlet. In order to compile and run the upcoming 
examples, you'll need to make sure that the following JARs are in your classpath: 

• org. restlet. jar (Restlet API) 



346 | Chapter 12: Frameworks for RESTful Services 



• com.noelios.restlet.jar (Noelios Restlet Engine core) 

• com.noelios.restlet.ext.net.jar (HTTP client connector based on JDK's HttpURLCon 
nection) 

All these are available in the lib directory of the Restlet distribution. Make sure that 
your Java environment supports Java SE version 5.0 or higher. If you really need to, 
you can easily backport the Restlet code to Java SE version 4.0 with Retro translator 
(http://retrotranslator.sourceforge.net/). 

Example 12-3. A Restlet client for Yahool's search service 

II YahooSearch. java 

import org. restlet. Client; 

import org . restlet . data . Protocol ; 

import org . restlet . data . Reference ; 

import org . restlet . data . Response; 

import org . restlet .resource . DomRepresentation ; 

import org. w3c.dom. Node; 

/** 

* Searching the web with Yahool's web service using XML. 
*/ 

public class YahooSearch { 

static final String BASEJJRI = 

" http : //api . search . yahoo . com/WebSearchService/Vl/webSearch " ; 

public static void main(String[ ] args) throws Exception { 
if (args. length != l) { 

System. err. println("You need to pass a term to search"); 
} else { 

// Fetch a resource: an XML document full of search results 
String term = Reference. encode(args[o]); 
String uri = BASE_URI + "?appid=restbook&query=" + term; 
Response response = new Client (Protocol. HTTP) .get(uri); 
DomRepresentation document = response. getEntityAsDomQ; 

// Use XPath to find the interesting parts of the data structure 
String expr = "/ResultSet/Result/Title"; 
for (Node node : document. getNodes (expr)) { 
System . out . println (node . getTextContent ( ) ) ; 

} 

} 

} 

} 

You can run this class by passing a search term as a command-line argument, just like 
with the Ruby example in Example 2-1. Here's a sample run: 

$ java YahooSearch xslt 

XSL Transformations (XSLT) 

The Extensible Stylesheet Language Family (XSL) 
XSLT Tutorial 



Restlet | 347 



This example demonstrates how easy it is with Restlet to retrieve XML data from a web 
service and process it with standard tools. The URI to the Yahoo! resource is built from 
a constant and the user-provided search term. A client connector is instantiated using 
the HTTP protocol. The XML document is retrieved with a method (get) whose name 
mirrors the method of HTTP's uniform interface. When the call returns, the program 
has the response entity as a DOM representation. As in the Ruby example, XPath is the 
simplest way to search the XML I retrieved. 

Also as in the earlier Ruby example, this program ignores the XML namespaces used 
in the result document. Yahoo! puts the entire document into the namespace 
urn: yahoo :srch, but I access the tags as, say, ResultSet instead of 
urn : yahoo : srch : ResultSet . The Ruby example ignores namespaces because Ruby's de- 
fault XML parsers aren't namespace-aware. Java's XML parsers are namespace-aware, 
and the Restlet API makes it easy to deal with namespaces correctly. It doesn't make 
much difference in a simple case like this, but you can avoid some subtle problems by 
handling documents in a namespace-aware way. 

Of course, saying urn:yahoo.srch:ResultSet all the time would get old pretty fast. The 
Restlet API makes it easy to associate a short prefix with a namespace, and then use 
the prefix in an XPath expression instead of the full name. Example 12-4 shows a variant 
of the document-handling code from the end of Example 12-3. This version uses name- 
space-aware XPath, so that Yahoo's ResultSet tag will never be confused with the 
ResultSet tag from some other namespace. 

Example 12-4. Namespace-aware version of the document handling code from Example 12-3 
DomRepresentation document = response. getEntityAsDomQ; 

// Associate the namespace with the prefix 'y' 

document . setNamespaceAware(true) ; 

document. putNamespace("y", "urn: yahoo: srch"); 

// Use XPath to find the interesting parts of the data structure 
String expr = "/y:ResultSet/y:Result/y:Title/text()"; 
for (Node node : document. getNodes(expr)) { 
System . out . println (node . getTextContent ( ) ) ; 

} 

Example 2-15 showed a second Ruby client for Yahoofs search service. That one re- 
quested a JSON representation of the search data, instead of an XML representation. 
Example 12-5 is the equivalent program for Restlet. It gets its JSON support from two 
additional JAR files, both included with Restlet: 

• org.restlet.ext.json_2.0.jar (Restlet extension for JSON) 

• org.json_2.0/org.json.jar (JSON official library) 



348 | Chapter 12: Frameworks for RESTful Services 



Example 12-5. A Restlet client for Yahool's JSON search service 

II YahooSearchDSON. java 

import org. json.DSONArray; 

import org. json. DSONObject; 

import org. restlet. Client; 

import org . restlet . data . Protocol ; 

import org . restlet . data . Reference ; 

import org . restlet . data . Response; 

import org. restlet. ext. json.HsonRepresentation; 

/** 

* Searching the web with Yahool's web service using DSON. 
*/ 

public class YahooSearchJSON { 
static final String BASE_URI = 

" http : //api . search . yahoo . com/WebSearchService/Vl/webSearch " ; 

public static void main(String[ ] args) throws Exception { 
if (args. length != l) { 

System. err. println("You need to pass a term to search"); 
} else { 

// Fetch a resource: a DSON document full of search results 
String term = Reference. encode(args[o]); 

String uri = BASE_URI + "?appid=restbook&output=json&query=" + term; 
Response response = new Client (Protocol. HTTP) .get(uri); 
DSONObject json = new DsonRepresentation(response.getEntityO) 
.toJsonObjectQ; 

// Navigate within the DSON document to display the titles 
DSONObject resultSet = json.getJSONObject("ResultSet"); 
DSONArray results = resultSet. getDSONArray("Result"); 
for (int i = 0; i < results. lengthQ; i++) { 

System. out. println(results.get]SONObject(i).getString("Title")); 

} 

} 

} 

} 

When you write a client against Yahool's service, you can choose the representation. 
Restlet supports both XML in the core API and JSON with an extension. As you'd 
expect, the only difference between the two programs is in processing the response. 
The DsonRepresentation class allows you to convert the response entity-body into an 
instance of DSONObject (contrast with Ruby's JSON library, which converted the JSON 
data structure into a native data structure). The data structure is navigated manually, 
since there's not yet any XPath-like query language for JSON. 

Writing Restlet Services 

The next set of examples is a little more complex. I'll show you how to design and 
implement a server-side application. I've implemented a subset of the bookmark man- 
agement application originally implemented with Ruby on Rails in Chapter 7. To keep 



Restlet | 349 



things relatively simple, the only features this application supports are the secure ma- 
nipulation of users and their bookmarks. 

The Java package structure looks like this: 

org 
restlet 
example 
book 
rest 
ch7 

-Application 
-ApplicationTest 
-Bookmark 
-BookmarkResource 

-BookmarksResource 

-User 

-UserResource 

That is, the class Bookmark is in the package org. restlet. example. book. rest. ch7, and 

so on. 

Rather than include all the code here, I'd like you to download it from the archive 
(http://www.oreilly.com/catalog/9780596529260), which contains all examples from 
this book. It's also available on restlet.org (http://www.restlet.org). If you've already 
downloaded Restlet, you don't have to do anything, since the examples for this section 
are shipped with Restlet, see src/org.restlet.example/org/restlet/example/book/rest. 

I'll start you off with some simple code in Example 12-6: the Application. main, which 
sets up the web server and starts serving requests. 

Example 12-6. The Application.main method: setting up the application 

public static void main(String. . . args) throws Exception { 
// Create a component with an HTTP server connector 
Component comp = new Component(); 
comp.getServersQ .add (Protocol. HTTP, 3000); 

// Attach the application to the default host and start it 
comp.getDefaultHost() .attach("/vl", new ApplicationQ); 
comp.startQ; 

} 

Resource and URI design 

Since Restlets impose no restrictions on resource design, the resource classes and the 
URIs they expose flow naturally from considerations of ROA design. There's no need 
to design around the Restlet architecture, the way the resources in Chapter 7 were 
designed around Rails's controller-based architecture. Figure 12-2 shows how incom- 
ing URIs are mapped to resources with a Router, and how resources are mapped onto 
the underlying restlet classes. 



350 | Chapter 12: Frameworks for RESTful Services 



Restlet compontent 



HTTP server 

D 



Application 



UserResource 











..'1 




±D Virtual host 




> 


Router 


-> 


BookmarkResource 
















BookmarkResource 



Bookmark 



Figure 12-2. Restlet architecture of the social bookmarking application 

To understand how these mappings are coded in Java, let's take a look at the 
Application class and its createRoot method (see Example 12-7). This is the equivalent 
of the Rails routes.rb file shown in Example 7-3. 

Example 12-7. The Application.createRoot method: mapping URI Templates to restlets 

public Restlet createRootQ { 

Router router = new Router(getContext()); 

// Add a route for user resources 

router. attach("/users/{username}", UserResource. class); 
// Add a route for user's bookmarks resources 

router. attach ("/users/{username}/bookmarks", BookmarksResource. class); 
// Add a route for bookmark resources 

Route uriRoute = router. attach("/users/{username}/bookmarks/{URI}", 

BookmarkResource. class); 
uriRoute . getTemplate( ) . getVariables ( ) 

.put ("URI", new Variable(Variable.TYPE_URI_ALL)); 

} 

This code runs when I create an Application object, as I do back in Example 12-6. It 
creates a clean and intuitive relationship between the resource class UserResource and 
the URI Template "/users/{username}". The Router matches incoming URIs against 
the templates, and forwards each request to a new instance of the appropriate resource 
class. The value of the template variables are stored in the request's attributes map 
(similar to the params map in the Rails example), for easy usage in the Resource code. 
This is both powerful and simple to understand, which is very helpful when you haven't 
seen the code for a few months! 



Restlet | 351 



Request handling and representations 

Suppose a client makes a GET request for the URI http://localhost:3000/vl/users/ 
jerome. I've got a Component listening on port 3000 of localhost, and an Application 
object attached to /vl. The Application has a Router and a bunch of Route objects 
waiting for requests that match various URI Templates. The URI path fragment 
"/users/jerome" matches the template "/users/{username}", and that template's 
Route is associated with the UserResource class: a rough equivalent to the Rails 
UsersController class. 

Restlet handles the request by instantiating a new UserResource object and calling its 
handleGet method. The UserResource constructor is reproduced in Example 12-8. 

Example 12-8. The UserResource constructor 
/** 

* Constructor. 

* (Sparam context 

* The parent context. 

* (Sparam request 

* The request to handle. 

* (Sparam response 

* The response to return. 

*/ 

public UserResource(Context context, Request request, Response response) { 
super(context, request, response); 

this.userName = (String) request. getAttributesQ .get("username") ; 
ChallengeResponse cr = request. getChallengeResponseQ; 
this. login = (cr != null) ? cr.getldentifierQ : null; 
this. password = (cr != null) ? cr.getSecretQ : null; 
this. user = findllserQ; 

if (user != null) { 

getVariants ( ) . add (new Variant (MediaType . TEXT_PLAIN ) ) ; 

} 

} 

By this time, the framework has set up a Request object, which contains all the infor- 
mation I need about the request. The username attribute comes from the URI, and the 
authentication credentials from the request's Authorization header. I also call 
f indUser to look up a user in the database based on the authentication credentials (to 
save space, I won't show the findUser method here). These are the jobs done by Rails 
filters in Chapter 7. 

After the framework instantiates a UserResource, it invokes the appropriate handle 
method on the resource object. There's one handle method for every method of HTTP's 
uniform interface. In this case, the last act of the Restlet framework is to call 
UserResource. handleGet. 

I don't actually define UserResource. handleGet, so the inherited behavior (defined in 
Restlet's Resource. handleGet) takes over. The default behavior of handleGet is to find 



352 | Chapter 12: Frameworks for RESTful Services 



the representation of the resource that best fits the client's needs. The client expresses 
its needs through content-negotiation. Restlet looks at the values of the Accept headers 
and figures out which "variant" representation is the most appropriate. In this case, 
there's only one representation format, so it doesn't matter what the client asks for. 
This is handled by the getVariants and getRepresentation methods. Back in the con- 
structor, I defined text/plain as the only supported representation format, so my im- 
plementation of the getRepresentation method is pretty simple (see Example 12-9). 

Example 12-9. UserResource. getRepresentation: building a representation of a user 
^Override 

public Representation getRepresentation(Variant variant) { 
Representation result = null; 

if (variant. getMediaTypeQ .equals(MediaType.TEXT_PLAIN)) { 

// Creates a text representation 

StringBuilder sb = new StringBuilderQ; 

sb.append(" W) j 

sb.append("User details\n"); 
sb.append(" \n\n"); 

sb. append ("Name: ") .append (this. user. getFullNameQ) . append ( '\n' ); 
sb. append ("Email: ") .append (this. user. getEmailQ) . append ( '\n ' ); 
result = new StringRepresentation(sb); 
} 

return result; 

} 

That's just one method of one resource, but the other resources, and the other HTTP 
methods of UserResource, work the same way. A PUT request for a user gets routed to 
UserResource. handlePut, and so on. As I mentioned earlier, this code is part of a com- 
plete bookmarking application, so there's much more example code available if you're 
interested in learning more. 

You should now understand how the Restlet framework routes incoming HTTP re- 
quests to specific Resource classes, and to specific methods on those classes. You should 
also see how representations are built up from resource state. You'll probably only have 
to worry about the Application and the Router code once, since a single router can work 
for all of your resources. 

Compiling, running, and testing 

The Application class implements the HTTP server that runs the social bookmarking 
service. It requires a classpath that contains the following JAR files: 

• org.restlet.jar 

• com.noelios.restlet.jar 

• com.noelios. restlet. ext. net. jar 

• org.simpleframework_3.1/org.simpleframework.jar 



Restlet | 353 



• com.noelios.restlet.ext. simple_3 . 1 .jar 

• com.db4o_6.1/com.db4o.jar 

All of these JAR files are included with the Restlet distribution, and we've listed them 
relative to the lib directory in your Restlet installation. Two things to notice: the actual 
web server work is handled by a very compact HTTP server connector based on the 
Simple Framework. Second, instead of making you set up a relational database, we 
persist our domain objects (users and bookmarks) with the powerful db4o object 
database. Once all the example files have been compiled, run org. restlet. exam 
pie. book. rest. ch7. Application, which acts as the server endpoint. 

The ApplicationTest class provides a client interface to the service. It uses the Restlet 
client classes described in the previous section to add and delete users and bookmarks. 
It does this through HTTP's uniform interface: users and bookmarks are created with 
PUT and deleted with DELETE. 

Run ApplicationTest. class from the command line and you'll get a message like this: 

Usage depends on the number of arguments: 

- Deletes a user : userName, password 

- Deletes a bookmark : userName, password, URI 

- Adds a new user : userName, password, "full name", email 

- Adds a new bookmark: userName, password, URI, shortDescription, 

longDescription, restrictftrue / false] 

You can use this program to add some users and give them bookmarks. Then you can 
view an HTML representation of the users' bookmarks by visiting the appropriate URIs 
in a standard web browser, such as http://localhost:3000/vl/users/jerome and so on. 

Conclusion 

The Restlet project delivered its final 1.0 version in early 2007. It took just more than 
12 months to develop, and the project now has a thriving development and user com- 
munity. The mailing list is friendly and welcoming to both new and experienced de- 
velopers. Noelios Consulting, the founder and main developing force behind the 
project, offers professional support plans and training. 

As of the time of writing, the 1.0 release is under maintenance, and a new 1.1 branch 
has been started. Future plans include submission of the Restlet API to the Java Com- 
munity Process (J CP) for standardization. There's also a higher-level API for RESTful 
web services in development, submitted by Sun Microsystems to the JCP and known 
as JSR 311. This higher-level API should make it easy to expose Java domain objects 
as RESTful resources. This will nicely complement the Restlet API, especially its 
Resource class. Noelios Consulting is part of the initial expert group and will directly 
support the future annotations in its Restlet engine. 



354 | Chapter 12: Frameworks for RESTful Services 



Django 



by Jacob Kaplan-Moss 

Django (http://www.djangoproject.com/) is a framework that makes it easy to develop 
web applications and web services in Python. Its design is very similar to Rails, though 
it makes fewer simplifying assumptions. You can apply the generic ROA design pro- 
cedure to turn a dataset into a set of RESTful resources and implement those resources 
directly in Django. 

I'll show you how I implemented a social bookmarking service in Django, along the 
lines of the Rails implementation in Chapter 7. Since this book isn't intended to be a 
Django tutorial, I'm leaving out most of the intermediary steps of Django development 
so I can focus on the parts that specifically apply to RESTful web services. If you're 
interested in learning more about Django, you should check out the free online Django 
Book (http://www.djangobook.com/) and the official Django documentation (http:// 
www.djangoproject.com/documentation/). 

Create the Data Model 

Most Django developers start by designing the data model. This corresponds to the 
first step of the generic ROA procedure, "Figure out the data set." The model is usually 
stored in a relational database using Django's object-relational mapping (ORM) tools. 
It's certainly possible to write RESTful services that don't use a database, but for the 
social bookmarking application a database makes the most sense. It's fairly straight- 
forward to translate the Rails migration from Example 7-1 into a Django model, as seen 
in Example 12-10. 

Example 12-10. The Django model (models.py) 

from datetime import datetime 

from django. db import models 

from django. contrib.auth. models import User 

class Tag(models. Model) : 

name = models. SlugField(maxlength=100, primary_key=True) 

class Bookmark(models. Model) : 



user = models. ForeignKey(User) 

url = models. URLField(db_index=True) 

short_description = models. CharField(maxlength=255) 

long_description = models. TextField(blank=True) 

timestamp = models. DateTimeField(default=datetime. now) 

public = models. BooleanFieldQ 

tags = models. ManyToManyField(Tag) 



There's a few subtleties and a lot of power squeezed into these few lines of code: 

• I chose to use the built-in Django User model, rather than create my own users 
table as the Rails example does. This has a few advantages, the biggest being that 



Django | 355 



the built-in User model will handle much of the authentication and authorization. 
For more information on this, see Chapter 12 of the Django Book (http://www.djan 
gobook.com/en/beta/chapterl2/) . 

• Django has no direct analog to the Rails acts_as_taggable plugin, so in the last line 
of the Bookmark definition I just define a many-to-many relation between 
Bookmark and Tag. 

• I'm defining the tag's name as a SlugField rather than a string. This is a Django class 
that automatically restricts tag names to those that can show up in a URI. This 
makes it easy to prohibit tags that contain spaces or other non-alphanumeric 
characters. 

• Most of the database indexes created explicitly in the Rails schema are automati- 
cally added by Django. In particular, slug fields and foreign keys automatically are 
given indexes. Notice, however, that I've had to explicitly specify db_index=True 
on the url field. That field won't get an index by default, but I want to search it. 

Define Resources and Give Them URIs 

The Rails implementation of the social bookmarking application exposes 1 1 resources. 
To keep the size of this section down, I'm only going to implement 4 of the 11: 

• A single bookmark 

• The list of a user's bookmarks 

• The list of bookmarks a user has tagged with a particular tag 

• The list of tags a user has used 

In particular, notice that I'm not exposing user accounts as resources. To use this service 
you'll need to pre-create some sample user accounts in the database. 

Ruby on Rails imposes simplifying assumptions that affect your URI design. Instead of 
defining resources, you define Rails controllers that expose resources at certain URIs. 
Django makes you design your URIs from scratch. Django's philosophy is that the URI 
is an important part of a web application's user interface, and should not be automat- 
ically generated. This fits in with the ROA philosophy, since a resource's only interface 
elements are its URI and the uniform interface of HTTP. 

Since Django forces you to design URLs explicitly, there's no "path of least resistance" 
as there is in Rails, so I'm able to make my Django URIs a little more compact and 
readable than the Rails URIs were. I'll modify the URI structure given for the Rails 
application in three ways: 

• The Django "house style" (as it were) is to always end URIs with a trailing slash. 
It's possible to go either way, of course, but to fit more with what Django developers 
expect I'll make all URIs include the trailing slash. That is, I'll use URLs like / users/ 
{username}/ instead of /users/{username}. 



356 | Chapter 12: Frameworks for RESTful Services 



• Rails's controller-based architecture make it convenient to expose bookmarks 
as /users/{username}/bookmarks/{URL}/. In Django it's just as convenient to use 
the more compact /users/{username}/{URL}/, so that's what I'll use. 

• Since I'm not exposing user accounts as resources, I can use URIs of the 
form /users/{username}/ for a different purpose. I'm going to expose my "book- 
mark list" resources there. 

• The Rails implementation uses POST to create a new bookmark as a subordinate 
resource of a bookmark list. I'll create new bookmarks using the other technique: 
by sending a PUT to /users/{username}/{URI}/, bypassing the bookmark list al- 
together. Rails had a problem with embedding a URI in another URI, so back then 
I exposed URIs like /users/{username}/bookmarks/{URI-MD5}. Here I can use the 
actual URIs themselves. 

I can easily use a Django URI configuration file to map these URIs to resources (Ex- 
ample 12-11). This is the equivalent of the routes. rb file in Example 7-3. It's a lot 
simpler, though, because Django doesn't try to make decisions about URI format for 
me. 

Example 12-11. A Django URI configuration: urls.py 

from django. conf.urls. defaults import * 
from bookmarks. views import * 

urlpatterns = patterns('', 

(r' A users/([\w- ]+)/$', bookmark_list), 
(r' A users/([\w-]+)/tags/$', tag_list), 
(r' A users/([\w-]+)/tags/([\w-]+)/\ tag_detail), 
(r' A users/([\w-]+)/(.*)\ BookmarkDetailQ), 

) 

The urls.py file is a small Python module that maps incoming URIs (represented as 
regular expressions) to the functions that handle the requests. Any groups in the regular 
expression are passed as arguments to the function. So if a request comes in for users/ 
jacobian/tags/python, Django will match it against the third regular expression, and 
call the tag_detail function with two arguments: "jacob" and "python". 

Since Django evaluates the URI patterns in order, I have to put the tag URIs before the 
bookmark URLs: otherwise, Django would interpret /users/jacobian/tags/ as a re- 
quest for a bookmark of the (invalid) URI tags. 

Of course, now I'm committed to writing four functions in a module called book 
marks, views. I won't leave you hanging, so let's move onto those functions. 

Implement Resources as Django Views 

Django interprets the Model-View-Controller pattern differently than Rails does. In 
Rails, to implement a resource's behavior under the uniform interface, you put code in 
a controller class. In Django, that code goes into the view. The Django FAQ (http:// 



Django | 357 



www.djangoproject.com/documentation/faq/) has more on this distinction, but I'll show 
you the implementation of two views: the read-only view function for the "bookmark 
list" resource and the read/write view class for the "bookmark" resource. 

The bookmark list view 

The bookmark list view function is a nice simple one to start with, because the "book- 
mark list" resource only responds to GET (see Example 12-12). Remember, I'm ex- 
posing bookmark creation through PUT, not through POST on the bookmark list the 
way the Rails implementation does. 

Example 12-12. First try at the bookmark list view 

from bookmarks. models import Bookmark 

from django.contrib.auth. models import User 

from django.core import serializers 

from django.http import HttpResponse 

from django. shortcuts import get_object_or_404 

def bookmark_list(request, username) : 

u = get_object_or_404(User, username=username) 
marks = Bookmark. objects. filter(user=u, public=True) 
json = serializers. serialize("json", marks) 
return HttpResponse( json, mimetype="application/json") 

The first step is to turn the argument username into a Django User object. The user 
name variable comes from the capture group in the regular expression from Exam- 
ple 12-11. It's everything between the parentheses in A user s/ ( [ \w- ] +) /$. Since a request 
for a non-existent user's bookmarks should return an HTTP response code of 404 ("Not 
Found"), I look up a user with Django's get_object_or_404() shortcut function. This 
will automatically raise a Http404 exception if the user doesn't exist, which Django will 
turn into an HTTP response code of 404. This serves the same purpose as the 
if_f ound method defined in the Rails application, way back in Example 7-9. 

In Chapter 7 1 kept the implementation short by using ActiveRecord's to_xml function 
to convert objects from the database (such as user accounts) into XML representations. 
I've used a similar trick here. Rather than represent lists of bookmarks as Atom feeds, 
I use Django's serialization API to turn database rows into JSON data structures (see 
http://www.djangoproject.com/documentation/serialization/). 

Django can also serialize database rows into a JSON data structure or an ActiveRecord- 
like XML representation: switching to the XML representation would be as easy as 
changing the serializer type in the third line of the view, and the mimetype in the last 
line. Django's default JSON output is relatively straightforward. Example 12-13 shows 
what it does to a row from my bookmarks table. 

Example 12-13. A JSON representation of a bookmark 
[{ 

"pk": "1", 



358 | Chapter 12: Frameworks for RESTful Services 



"model" : "bookmarks. bookmark", 
"fields": { 

"tags": ["example"], 

" url" : " htt p : \/\/example . com\/ " , 

"timestamp": "2007-01-30 21:35:23", 

"long_description" : "", 

"user": 1, 

"short_description" : "Example", 
"public": true 

} 

}] 

The bookmark_list implementation from Example 12-12 will work, but it's a bit naive. 
It returns all of a user's bookmarks every time it's called, and that will chew up both 
database resources and bandwidth. Example 12-14 shows an implementation that adds 
support for conditional GET (see "Conditional GET" in Chapter 7). Adding handling 
for Last-Modified and If-Modif ied-Since does make this view more complex, but the 
bandwidth savings will make it worth the effort. 

Example 12-14. Second try at the bookmark list view 
import datetime 

from bookmarks. models import Bookmark 
from django.contrib.auth. models import User 
from django.core import serializers 
from django.http import * 

from django. shortcuts import get_object_or_404 

# Use the excellent python-dateutil module to simplify date handling. 

# See http://labix.org/python-dateutil 
import dateutil. parser 

from dateutil. tz import tzlocal, tzutc 

def bookmark_list(request, username): 

u = get_object_or_404(User, username=username) 

# If the If-Modified-Since header was provided, 

# build a lookup table that filters out bookmarks 

# modified before the date in If-Modified-Since. 
lookup = dict(user=u, public=True) 

lm = request . META . get ( "HTTP_I F_MODI FI ED_SINCE " , None) 
if lm: 
try: 

lm = dateutil. parser. parse(lm) 
except ValueError: 

lm = None # Ignore invalid dates 
else: 

lookup [ 'timestamp gt'] = lm.astimezone(tzlocal()) 

# Apply the filter to the list of bookmarks, 
marks = Bookmark. objects. filter(**lookup) 

# If we got If-Modified-Since but there aren't any bookmarks, 

# return a 304 ("Not Modified") response. 



Django | 359 



if lm and marks. count() == 0: 

return HttpResponseNotModifiedQ 

# Otherwise return the serialized data... 
json = serializers.serialize("json", marks) 

response = HttpResponse(json, mimetype="application/json") 

# ... with the appropriate Last-Modified header, 
now = datetime.datetime.now(tzutc()) 

response["Last-Modified"] = now.strftime("%a, %d %b %Y %H:%M:%S GMT") 
return response 

There's a number of other improvements that could be made here — most notably the 
ability to show private bookmarks to authenticated users — but you've already seen 
these features in Chapter 7. I'll leave porting them to Django as exercises, and forge 
ahead. 

The bookmark detail view 

The second view I'll show you has more moving parts. A "bookmark list" resource only 
responds to GET, but a "bookmark" resource must handle three HTTP methods. GET 
on a bookmark retrieves a representation of the bookmark, PUT creates or updates a 
bookmark, and DELETE removes a bookmark. Since we don't want users modifying 
each others' bookmarks, the bookmark resource needs to take authentication into ac- 
count. 

The most obvious way to sketch out the bookmark_detail view is with a series of if 
clauses: 

def bookmark_detail(request, username, bookmark_url) : 
if request. method == "GET": 

# Handle GET 

elif request. method == "POST" : 

# Handle POST 

elif request. method == "PUT": 

# Handle PUT 

elif request. method == "DELETE": 

# Handle DELETE 

However, this is unelegant and will quickly lead to an unwieldy view. Instead, I'll take 
advantage of Python's "duck typing" and implement the bookmark detail view as a 
callable object. In Python, functions are first-class objects, and the syntax of a function 
call (object(argument)) is transformed into a method call on the object 

(object . call (argument)). This means that any object can be called like a function, 

if it defines the call method. I'm going to take advantage of this trick by imple- 
menting the bookmark detail as a class with a call method. 

This is why the last line of Example 12-1 1 looks different from the other three. The first 
three tie regular expressions to Python function objects: bookmark_list and the like. 
The last one ties a regular expression to a custom object that happens to implement 
call . The call implementation will do some preliminary work, check the 



360 | Chapter 12: Frameworks for RESTful Services 



HTTP method of the incoming request, and dispatch to an appropriate action function 
(see Example 12-15). 

Example 12-15. The bookmark detail view, part 1: dispatch code 
class BookmarkDetail: 

def call (self, request, username, bookmark_url) : 

self. request = request 

self .bookmark_url = bookmark_url 

# took up the user and throw a 404 if it doesn't exist 
self. user = get_object_or_404(User, username=username) 

# Try to locate a handler method, 
try: 

callback = getattr(self , "do_%s" % request. method) 
except AttributeError: 

# This class doesn't implement this HTTP method, so return 

# a 405 ("Method Not Allowed") response and list the 
#allowed methods. 

allowedjnethods = [m.lstrip("do_") for m in dir(self) 

if m.startswith("do_")] 
return HttpResponseNotAllowed(allowed_methods) 

# Check and store HTTP basic authentication, even for methods that 

# don't require authorization, 
self .authenticate() 

# Call the looked-up method 
return callbackQ 

The BookmarkDetail. call method checks the HTTP method of the incoming re- 
quests, and dispatches each one to an appropriate method of the form do_<METH0D>. For 
instance, a GET request is dispatched to do_GET. Rails does something similar behind 
the scenes, turning a GET request into a call to MyController#show. 

The BookmarkDetail class also needs to handle HTTP basic authentication, so let's take 
a look at that now. In a real application, these functions would go into a superclass to 
be used by every view that required authentication. Think back to Chapter 7, and the 
way I put the must_authenticate Rails filter into the base ApplicationController class 
(see Example 12-16). 

Example 12-16. The bookmark detail view, part 2: authentication code 
from django.contrib.auth import authenticate 

class BookmarkDetail: 

# ... 

def authenticate(self ) : 

# Pull the auth info out of the Authorization header 
auth_info = self .request. META.get("HTTP_AUTHORIZATION", None) 
if auth_info and auth_info.startswith("Basic "): 



Django | 361 



basic_info = auth_info.lstrip("Basic ") 

u, p = auth_info.decode("base64").split(":") 

# Authenticate against the User database. This will set 

# authenticated_user to None if authentication fails. 

self .authenticated_user = authenticate(username=u, password=p) 
else: 

self .authenticated_user = None 

def forbidden(self ) : 

response = HttpResponseForbiddenQ 

response[ "WWW-Authenticate"] = 'Basic realm= " Bookmarks " ' 
return response 

Now we can check self .authenticated_user within the individual do_{METH0D} meth- 
ods. I've also written a forbidden () helper that sends an HTTP 401 (Forbidden) re- 
sponse with the correct WWW-Authenticate header. 

Now I'll implement the "bookmark" resource's response to each HTTP method it ex- 
poses. GET is the simplest, so let's start there. Example 12-17 shows the implementa- 
tion of do_GET. It illustrates the same concepts as the bookmark list's response to GET, 
back in Example 12-14. The only major difference is that we enforce the privacy of 
private bookmarks. 

Example 12-1 7. The bookmark detail view, part 3: GET 

def do_GET(self): 

# Look up the bookmark (possibly throwing a 404) 
bookmark = get_object_or_404(Bookmark, 

user=self .user, 
url=self .bookmark_url 

) 

# Check privacy 

if bookmark. public == False and self. user != self .authenticated_user: 
return self .forbiddenQ 

json = serializers.serialize("json", [bookmark]) 
return HttpResponse( json, mimetype="application/json") 

Next up is PUT (see Example 12-18). This method needs to take an incoming repre- 
sentation of a bookmark's state, and use it to either create a new bookmark or update 
an existing one. The incoming representation is available as 
self .request. raw_post_data, and I use the Django serialization library to turn it from 
a JSON data structure to a Django database object. 

Example 12-18. The bookmark detail view, part 4: PUT 
def do_PUT(self): 

# Check that the user whose bookmark it is matches the authorization 
if self. user != self .authenticated_user: 

return self .forbiddenQ 

# Deserialize the representation from the request. Serializers 



362 | Chapter 12: Frameworks for RESTful Services 



# work the lists, but we're only expecting one here. Any errors 

# and we send 400 ("Bad Request"), 
try: 

deserialized = serializers.deserialize("json", 

self . request . raw_post_data) 
put_bookmark = list(deserialized) [o] .object 
except (ValueError, TypeError, IndexError): 
response = HttpResponseQ 
response. status_code = 400 
return response 

# Look up or create a bookmark, then update it 
bookmark, created = Bookmark. objects. get_or_create( 

user = self. user, 

url = self .bookmark_url, 

) 

for field in [ "short_description", "long_description", 
"public", "timestamp"] : 
new_val = getattr(put_bookmark, field, None) 
if new_val: 

setattr(bookmark, field, new_val) 
bookmark. save() 

# Return the serialized object, with either a 200 ("OK") or a 201 

# ("Created") status code. 

json = serializers.serialize("json", [bookmark]) 

response = HttpResponse(json, mimetype="application/json") 

if created: 

response. status_code = 201 

response["Location"] = "/users/%s/%s" % \ 

(self .user. username, bookmark. url) 

return response 

After all that, DELETE (Example 12-19) looks very simple. 

Example 12-19. The bookmark detail view, part 5: DELETE 

def do_DELETE(self ) : 

# Check authorization 

if self. user != self .authenticated_user: 
return self .forbiddenQ 

# Look up the bookmark... 

bookmark = get_object_or_404(Bookmark, 
user=self .user, 
url=self .bookmark_url 

) 

# . . . and delete it. 
bookmark. delete () 

# Return a 200 ("OK") 
response = HttpResponseQ 
response. status_code = 200 
return response 



Django | 363 



Further directions 

The tag views (and all the other interesting features like bundles, etc.) will follow a 
similar pattern. In fact, with a little work, this BookmarkDetail class could be refactored 
into a more general purpose resource class for handling many different types of objects. 

Conclusion 

Django isn't just a framework for handling HTTP requests. Like Rails, it contains a lot 
of sub-libraries that handle common problems in web application and web service 
design. You've seen the object-relational mapper that works like Rails's ActiveRecord, 
the built-in User model, and the serialization of model objects into JSON representa- 
tions. Django has many other libraries, including a comment model and a tool for 
generating syndication feeds. Though it's mostly used for web applications, Django 
makes an excellent base for Python implementations of RESTful web services. 



364 | Chapter 12: Frameworks for RESTful Services 



APPENDIX A 

Some Resources for REST and Some 

RESTful Resources 



The World Wide Web is the largest distributed application ever created, consisting of 
billions of resources. I've spent this book showing you how to seamlessly integrate a 
few new resources into this global application. Now I'm going to create a few links of 
my own. This appendix is a bit of hypermedia that connects this book to other discus- 
sions of REST, and to real live web services. 



Standards and Guides 

These are just a few of the sites that have helped me make sense of the programmable 
web. 



HTTP and URI 

• The HTTP standard (RFC 2616) (http://www.w3.org/Protocols/rfc2616/ 
rfc2616.html). 

• The URI standard (RFC 3986) (http://www.ietf.org/rfc/rfc3986.txf). 

• The WEBDAV standard (RFC 2518) (http://www.webdav.org/specs/rfc2518.html), 
if you're interested in extensions to HTTP's uniform interface. 

• The Architecture of the World Wide Web introduces concepts like resources, rep- 
resentations, and the idea of naming resources with URIs (http://www.w3.org/ 
2001/tag/webarch/) . 

• Universal Resource Identifiers — Axioms of Web Architecture (http://www.w3.org/ 
Designlssues/ Axioms) . 



365 



RESTful Architectures 



• The Fielding dissertation: Architectural Styles and the Design of Network-Based 
Software Architectures (http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm). 

• The very active rest-discuss mailing list (http://tech.groups.yahoo.com/group/rest- 
discussf). 

• The RESTwiki (http://rest.blueoxen.net/). 

• Joe Gregorio's "REST and WS (http://bitworking.org/news/125/REST-and-WS)" 
compares the technologies of REST to those of the WS-* stack while showing how 
to create a RESTful web service interface; if you don't care about the comparison 
part, try "How to create a REST Protocol (http://bitworking.org/news/How_to_cre 
ate_a_REST_Protocol) " by the same author. 

• Joe Gregorio has also written a series of articles on REST for XML.com (http:// 
www.xml. com/pub /au/225) . 

• Duncan Cragg's The REST Dialogues (http://duncan-cragg.org/blog/post/getting- 
data-r est- dialogues/): This series of weblog entries is a thought experiment that re- 
envisions an RPC-style application as a web of interconnected resources (though 
it doesn't use those terms). 

• Paul Prescod's Common REST Mistakes (http://www.prescod.net/rest/mistakes/). 

Hypermedia Formats 

• The XHTML standard (http://www.w3.org/TR/xhtmll/), which is just a set of small 
changes on top of the HTML standard (http://www.w3.org/TR/html4) . 

• The microformats web site (http://microformats.org/) and wiki (http://microfor 
mats.org/wiki/). 

• The URI Templates draft (http://bitworking.org/projects/URI-Templates/). 

• The Web Applications 1.0 standard (http://www.whatwg.org/specs/web-apps/cur 
rent-work/), which forms the basis of the forthcoming HTML 5. I think the most 
interesting part of the standard is Web Forms 2.0 (http://www.whatwg.org/specs/ 
web-forms/current-work/), which greatly improves HTML's hypermedia 
capabilities. 

• The WADL standard, maintained at the development site for the Java WADL cli- 
ent (http://wadl.dev.java.net/) . 

Frameworks for RESTful Development 

• As I showed in Chapter 7, Ruby on Rails (http://rubyonrails.org/) makes it easy to 
expose RESTful resources. The second edition of Agile Web Development with 
Rails by Dave Thomas et al. (Pragmatic Programmers) is the canonical reference 
for Rails. 



366 | Appendix A: Some Resources for REST and Some RESTful Resources 



• David Heinemeier Hansson's keynote at Railsconf 2006 (http: //www. s crib eme 
dia.org/2006/07/09/dhh/) shows how Rails moved from a REST-RPC philosophy 
to one based on RESTful resources. 

• The Restlet framework for Java (http://www.restlet.org/) can model any RESTful 
architecture, not just the resource-oriented web services covered in this book. As 
I write this, a Java standard for RESTful web services has just begun development 
as JSR 3 1 1 (http://jcp.org/en/jsr/detail ?id=3 11). 

• Django for Python (http://www.djangoproject.com/): As of the time of writing, a 
free Django book (http://www.djangobook.com/) was in development. 

Weblogs on REST 

The REST community is full of eloquent practitioners who argue for and explain 
RESTful architectures on their weblogs. I'll give out links to just a few. You can find 
more, in true REST fashion, by following links. 

• Mark Baker (http://www.markbaker.ca/blog/) 

• Benjamin Carlyle (http://www.soundadvice.id.au/blog/) 

• Joe Gregorio (http://bitworking.org/news/) 

• Pete Lacey (http://wanderingbarque.com/nonintersecting/) 

• Mark Nottingham (http://www.mnot.net/blog/) 

• RESTful Web Services co-author Sam Ruby (http://www.intertwingly.net/blog/) 

Services You Can Use 

The web is full of RESTful resources, but some are more technically interesting than 
others. Behind every RSS and Atom feed, behind every weblog and podcast, is a RESTful 
resource: an addressable, stateless "target of a hypertext link" that's full of links to other 
resources. You might think it's cheating to count these as RESTful web services. It's 
not. The world is full of Big Web Services that could be, or have been, replaced with a 
set of syndication feeds. 

But you're probably not looking in this appendix for a list of interesting syndication 
feeds. You're probably interested in services that have more architectural meat to them. 
In this section I focus on RESTful web services that let the client create and modify 
resources. Read-only resource-oriented services are very common and fairly well un- 
derstood. So are read/write REST-RPC hybrids. Read/write RESTful services are rela- 
tively rare, and those are the ones I want to showcase. 



Services You Can Use | 367 



Service Directories 



If you're trying to write a client, and you want to see whether there's a web service that 
does what you need, I refer you to one of these directories. You might find a RESTful 
resource-oriented service that works for you, or you might find an RPC-style or REST- 
RPC service you can use. 

• ProgrammableWeb (http://programmableweb.com/) is the most popular web serv- 
ice directory. It tracks both the APIs that make up the programmable web, and the 
mashups that combine them. Its terminology isn't as exact as I'd like (it tends to 
classify REST-RPC hybrids as "REST" services), but you can't beat it for variety. 

• By contrast, servicereg.com (http://www.servicereg.com/) hardly has any services 
registered with it. But I think it's got promise, because it's not just a web service 
directory: it's also a web service. The list of web services is exposed as a "collection" 
resource that speaks the Atom Publishing Protocol. 

• MicroApps (http://microapps.org/) focuses on RESTful applications which are de- 
signed to be used as components in other applications, like Amazon S3. 

Read-Only Services 

As I said earlier, read-only RESTful services are very common and not very interesting 
architecturally. I'll just give three examples of read-only services that do especially in- 
teresting things. You can find many more examples on ProgrammableWeb. 

• irrepressible. info (http://irrepressible.info/api) exposes a set of syndication feeds 
that help disseminate material censored by various governments. 

• Audioscrobbler (http://www.audioscrobbler.net/data/webservices/) exposes a large 
dataset about music and people who listen to it. 

• The Coral Content Distribution Network (http://www.coralcdn.org/) offers a sim- 
ple interface to a distributed cache of web resources. It would also have been 
RESTful to have implemented this service with the interface of an HTTP proxy 
cache, but resource-oriented designs are more popular. 

Read/Write Services 

The Atom Publishing Protocol (covered in Chapter 9) is the most popular model for 
read/write RESTful services. There's a partial list